Studies on the Vertebrate Head.

Part I. Fish.

By

G. It. de Beer, M.A., B.Sc, F.L.S.,Fellow of Merton College, Demonstrator in Zoology and Comparative

Anatomy, Oxford University.

With 42 Text-figures.

CONTENTS.PAGE

1 . I N T R O D U C T I O N . . . . . . . . . 2 8 8

2 . S E L A C H I A N S . . . . . . . . . 2 8 9

( a ) T h e V e n a e C a p i t i s 2 8 9(b) T h e A r t e r i e s 2 9 8

(i) D e v e l o p m e n t 2 9 8(i i ) R e l a t i o n s of t h e A r t e r i e s . . . . . 3 0 1

3 . T E L E O S T O M E S . . . . . . . . . 3 0 4

(a) A m i a c a l v a . . . . . . . . 3 0 4(i) T h e V e n a e C a p i t i s 3 0 4

(i i ) T h e R e l a t i o n s of t h e A r t e r i e s . . . . 3 0 9(i i i ) D e v e l o p m e n t of t h e A r t e r i e s . . . . 3 1 5

( J i ) S a l m o t r u t t a 3 1 6(i) T h e V e n a e C a p i t i s 3 1 6

(i i ) T h e A r t e r i e s 3 1 8(c) Cottus bubal is 319

The Venae Capitis 3 1 9

4 . D I P N O I 3 2 0

C e r a t o d u s f o r s t e r i . . . . . . . 3 2 0(i) T h e V e n a e C a p i t i s 3 2 0

(ii) T h e A r t e r i e s 3 2 3

5 . C Y C L O S T O M E S . . . . . . . . . 3 2 4

P e t r o m y z o n . . . . . . . . 3 2 4(i) T h e V e n a e C a p i t i s 3 2 4

(ii) T h e A r t e r i e s 3 2 7

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PAGE6 . C O N S T A N T R E L A T I O N S . . . . . . . . 3 2 8

7 . I N C O N S T A N T R E L A T I O N S . . . . . . . 3 2 9

( a ) T h e M a n d i b u l a r V e s s e l 3 2 9( b ) T h e H y o m a n d i b u l a r 3 3 3

8 . C O N C L U S I O N S . . . . . . . . . . 3 3 7

9 . S U M M A R Y 3 3 9

1 0 . L I S T O F L I T E R A T U R E C I T E D . . . . . . . 3 3 9

1. INTRODUCTION.

THE structure of the head of the vertebrate might appearto be more suitable as a subject for a text-book than for apaper. There exist innumerable and admirable descriptionsof the structure of the head of various forms in books and papers,but it appeared to the writer that detailed comparisons betweenrelated forms belonging to one group not only constituted aless threadbare subject for investigation, but also might revealsome points either unknown, unexplained, or not well under-stood. Accordingly the present work aims at a descriptionof few new structures, but at the comparison of the relationswhich these structures bear to one another in related forms.The present communication deals with the most lowly groups,the Fish and Cyclostomes.

The forms investigated were: Scyllium canicula ,Squalus acanth ias , P r i s t i u r u s melanostomus,He te rodon tus phil ippi, Torpedo ocel la ta , Amiacalva, Salmo t r u t t a , Cottus bubal is , Ceratodusforsteri , Petrornyzon planeri , P. f luviat i l is .

The relations of the eye-muscles and the oculomotor nervehave been dealt with elsewhere (14). Here the structurescalling for consideration are the arteries and veins, and therelations which they bear to various nerves and cartilages.

The work was done in the Department of Zoology andComparative Anatomy, University Museum, Oxford. I desireto acknowledge my indebtedness to Professor Goodrich, P.B.S.,for his encouragement during the work.

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2. SELACHIANS.

(a) The Venae Capitis.The external jugular veins of the adult dog-fish are, as is

well known, not the primitive veins of the head region (seeHochstetter, 19 and 20 ; Babl, 26). The primitive vein, orvena capitis medialis as it is termed from its position, is theonly cranial vein in ScyIlium, Stage I (Text-fig. 1). In

TEXT-FIG,

Selected transverse sections through Scy l l i un i oan icu la ,Stage I, in the regions of (a) trigeminal, (6) facial, (c) auditornerve, (d) auditory vesicle, (e) glossopharyngeal, (/) vagus,(g) first postotic myotome.

EXPLANATION OF LETTERING OF PIGS. 1-42.

a, auricle; ah, abducens; aba, afferent branchial artery; abaltfirst branchial artery; ac, anterior cardinal; am, anteriorcerebral artery ; acr, arteria centralis retinae ; aha, afferenthyoid artery; anteba, anterior efferent branchial artery; apa,afferent pseudobranchial artery; as, av, auditory sac ; on,auditory nerve ; auc, auditory capsule ; ba, branchial arch ; bas,third branchial arch ; baa, basilar artery ; hue, ramus buccalisfacialis; ca, cerebral artery; cc, cross commissure between efferentbranchial arteries; ch, ceratohyal; cv, connecting vessel between

290 G. R. DB BEER

transverse sections of embryos it is seen to lie median to the'trunks of all the dorsal cranial nerves, i. e. trigeminal, facial,glossopharyngeal, and vagus ; and in the region of the auditory

hypobranchial artery and ductus cuvieri; eim, connecting vesselbetween facial vein and vena capitis medialis ; da, dorsal aorta ;do, ductus cuvieri; e, eye; eb, epibranchial cartilage; ebai_h,first to fifth efferent branchial artery; ec, external carotid ; eha,efferent hyoid artery; ehv, hyoid vein; en, endostyle; epa, efferentpseudobranchial artery ; exr, external rectus ; exv, extra vein ;,/', facial nerve ; fb, forebrain ; fgl, foramen for glossopharyngeal;fhmf, foramen for hyomandibular branch of facial; foe, foramenfor oculomotor ; forba, foramen for orbital artery ; fv, facial vein ;cjl, glossopharyngeal nerve; ?«]_<, gill-slits 1 to 4; h, hypophysis ;ha, hypobranchial artery ; Kb, hind-brain ; hm, hyomandibular ;hmc, cut dorsal portion of hyomandibular ; hmf, ramus hyo-mandibularis facialis ; hmnis, hyomandibular muscle ; hv, hypo-physial vein; i, inf undibulum; ic, internal carotid; infr,inferior rectus; indb, inferior oblique; inos, inferior orbitalsinus ; inr, internal rectus ; lev, lateral cutaneous vein ; ma,mandibular artery ; max, ramus maxillaris trigeinini; me,Meckel's cartilage ; md, ramus mandibularis trigemini; mdmx,maxillomandibular branch of the trigeminal; mexf, ramusmandibularis externus facialis ; minf, ramus mandibularis in-ternus facialis ; ms, mandibular sinus ; my, myotome; inyd,dorsal part of myotome ; vvyv, ventral part of myotome ; n, noto-chord ; nic, notch for internal carotid ; nrpf, notch for ramuspalatinus facialis ; oc, oculomotor nerve ; ocs, occipital sinus ;ol, lateral occipital cartilage ; on, optic nerve ; op, operculum ;ophma, ophthalmica magna artery ; orba, orbital artery ; os,orbital sinus ; pa, pathetic nerve ; pant, pila antotica ; papa,primary afferent pseudobranchial artery ; pasc, processus ascen-dens ; pb, pharyngobranchial cartilage ; pc, parachordal; pun,,posterior cerebral artery ; pev, posterior cardinal vein ; peba,posterior efferent branchial artery ; po, processus oticus of thepterygoquadrate ; poc, polar cartilage ; pohm, processus oticushyomandibularis ; ppb, processus palatobasalis ; ppq, palato-pterygoquadrate; pr, profundus nerve; ps, pseudobranch;r, rostrum ; re, ramus communicans between the facial and vagus ;rop, ramus ophthalmicus profundus; ros, rami ophthalmioisuperiores ; ros VII, ramus ophthalmicus superficial facialis ;rpf, ramus palatinus facialis ; rvel, rudiment of the vena capitislateralis; «,, 2> s» first, second, third somites; sh, stylohyal;si, sphenolateral cartilage ; sn, sinus venosus ; sp, spiracle ; sip,styloid process ; snob, superior oblique ; siios, superior orbitalsinus ; sur, superior rectus ; sv, secondary vein ; te, trabeculacranii; tcom, trabecula communis ; th, thymus ; tp, trabecularplate ; Ir, trigeminal nerve ; v, vagus nerve ; va, ventral aorta ;vahm, vein accompanying the hyomandibular nerve ; zap, veinaccompanying the profundus nerve; vea, velar artery; vev,velar vein; vl, lateralis branch of vagus; vn, ventricle; w,, ,JS 3,first ventral nerve-root, second and third ditto ; zs, rudiment ofotic process of the hyomandibular.

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sac it runs between the latter and the wall of the brain. Theserelations will be obvious from Text-fig. 1, and show that theterm vena capitis medialis is accurately descriptive.

Text-fig. 2 is a reconstruction of a slightly older embryo(Stage J) from the left side; the vena medialis can be seenpassing on the inside of the nerves and of the auditory sac.Just behind the oculomotor nerve the veins of each side com-municate by the hypophysial vein. Between the glosso-

TEXT-FIG. 2.

Reconstruction of Stage J (Scyllium) from the left side.

pharyngeal and vagus there appears the rudiment of a newvein, external to the medialis, which will form part of the venacapitis lateralis of the adult. In Text-fig. 3 (Stage K) the venacapitis lateralis is present external to the facial, but it has notyet joined with the posterior portion between the glossopharyn-geal and the vagus. This stage is shown in section in Text-fig. 4 ;there is only the medialis in the region of the glossopharyngeal.It is important to observe that in the region of the vagus thenew vein is at this stage still internal to the nerve. For thisreason it is here denoted ' secondary vein ' until it acquiresits definitive external position.

292 G. R. DE BEER

TEXT-FIG. 3.

Reconstruction of Stage K (ScyIlium) from the dorsal side.

TEXT-FIG. 4.

Selected sections through S c y l l i u m , Stage K, in the regionsof (a) trigeminal, (6) facial, (c) auditory nerve, {d) auditory vesicle,(e) glossopharyngeal, •(/) slightly behind (e), (g) vagus, (7i) firstpostotic myotome.

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TEXT-FIG. 5.

Reconstruction of 21 mm. Heterodontus pliilippi from theleft side.

TEXT-FIG. 6.

Selected sections through Stage 0, Scyllium, in the regions of(a) trigeminal, (6) facial, (c) and (d) auditory capsule, (e) glosso-pharyngeal, (/) vagus.

294 G. R. DE BEER

Text-fig. 5 is taken from* a reconstruction of an embryoof Hete rodontus in Stage L. The lateralis has joined themedialis just in front of the facial and in front of and behindthe glossopharyngeal. By looping round, and the branchesjoining on the outer side, the first branch of the vagus has been

TEXT-FIG. 7.

Keconstruction of Scy l l i um from the dorsal side (the brainis removed), showing the vena capitis lateralis.

enclosed by the lateralis. The medialis is still the principalvein. The secondary vein has become lateralis.

Anteriorly the medialis forms the orbital sinuses. A moreventrally situated sinus (inferior orbital sinus) lies median tothe oculomotor nerve ; the superior orbital sinus, when itextends forwards, will lie external to this nerve.

The medialis (and its anterior prolongation the inferior

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orbital sinus) lies internal to the ventral cranial nerves (oculo-motor, pathetic) except the abducens. The latter issues fromthe brain very nearly in the median line, and consequentlythe vein must lie external to it. That this is secondary issuggested by the relations of the postotic nerve-roots to themedialis as shown in Text-fig. 1 g, where the nerve lies dorsaland external to the vein.

At a later stage (Text-fig. 6) the medialis dwindles away, beingcompressed between the developing auditory sac and the brain.

TEXT-FIG. 8.

Reconstruction of S c y l l i u m (26 mm.) from the left side.

Text-fig. 7, which is a reconstruction from the dorsal side,shows how the medialis ends blindly posteriorly ; the lateralisis now the functional vein of the head. By looping round,nearly all the branches of the vagus are enclosed in the vein.

Anteriorly the superior orbital sinus connects with themedialis behind the maxillo-mandibulary branch of the.trigeminal, forming a lateralis in this region. The hypophysialvein comes off from the medialis median to the trigeminal.

At 26 mm. ( S c y l l i u m , Text-fig. 8) the lateralis is fullyformed and lies external to all the branches of the vagus,and indeed to all the nerves except the ophthalmic branches ofthe trigeminal and facial. The only portion of the medialis

296 G. R. DE BEER

remaining lies median to the trigeminal, and from it thehypophysial vein arises. The trigeminal and facial nervesthus come to be separated by a vein, and it is part of thisvein which enters the skull in Hete rodontus betweenthese tAvo nerves (see 13).

At 33 mm. (ScyIlium, Text-fig. 9) cartilage is developed.The lateralis lies outside the auditory capsule and dorsal to thehyomandibular but ventral and internal to the hyomandibular

TEXT-KIO. 9.

suos ros

eha \hm

Reconstruction of Scyllium (33 mm.) from the left side.

muscle. The hypophysial vein runs inwards anterior to thepila antotica joining the sphenolateral cartilages with theparachordals, and under the tip of the notochord. Text-fig. 10 gives transverse sections through the regions of the nervesat the stage from which Text-fig. 9 was reconstructed.

Text-fig. 11 is a reconstruction from the dorsal surface ofa 70 mm. embryo of Heterodontus . As well as thehypophysial vein, there are two others which pierce the skull.One of these pierces the skull slightly anterior and ventral to theoptic foramen and runs for a short distance along the floor,

VERTEBRATE HEAD 297

which it pierces, and then enters the olfactory capsule. Theother pierces the skull by the same foramen through -whichthe ramus ophthalmicus profundus passes.

In tracing the development of the venous system of thehead it has been shown that the primitive vein becomessquashed by the development of the auditory sac. It isreplaced by the definitive lateral vein, and in two ways. First,

TEXT FIG

Selected sections through Scyllixim (33 mm.) in the regions of(a) trigemitml, (6) facial, (c) and (d) auditory capsule, (e) hyo-mandibular, (/) vagus.

rudiments of the lateralis appear, which later become connectedup. Next this vein laps round the vagus branches by makingsmall and progressive encroachments. Eventually that portionwhich was median to the vagus disappears. The possibility offorming a lateralis vein is conditioned by the fact that in thehead region the myotomes are either small or absent. As soonas the postotic myotomes are reached the vein lies median tothem.

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(b) The Arteries.(i) Development.—The early stages of development of

the arteries have been described by Dohrn (15), Eaffaele (27),and others, to whom references will be found in Hochstetter's

TEXT-FIG. 11.

Reconstruction of H e t e r o ' d o n t u s (70 mm.) from the dorsalside, the roof of the skull removed.

(20) work. Other workers who have investigated the condi-tions of the arteries in Selachians include Allis (6), Ayers (10),Carazzi (11), Hyrtl (21), and Parker (24).

In an 11mm. embryo of Heterodontus (Text-fig. 12a)

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the condition of the arteries is simple. There are a dorsaland a ventral aorta communicating through each arch by asingle branchial vessel.

At 21mm. (Text-fig. 126) the spiracular anterior demi-branch has appeared. The vessel in the hyoid arch has splitinto two, an afferent and an efferent. The efferent hyoideanartery is connected with the mandibular artery ventral to thedemibranch by a cross commissure running beneath the spiracle.The spiracle is much reduced in size and has moved dorsally.The vessel in the first branchial arch has likewise split intotwo, but both portions connect both with the dorsal and ventralaortae. Anteriorly the dorsal aorta is continued forwards asthe internal carotid. This gives off an artery running alongtogether with the optic nerve into the eyeball (the arteriacentralis retinae), a small artery to the olfactory capsule,and then curves back, running under the floor of the brain asthe arteria basilaris.

At 25 mm. (Text-fig. 12 c) there is little change in the hyoidarch, but the efferent hyoid artery gives off a branch passingback beneath the first gill-slit towards the first efferent branchialartery, which at this stage it does not yet reach. Where theefferent artery from the spiracular demibranch joins theinternal carotid an artery is given off which runs into the eye,the arteria ophthalmica magna. Slightly further posteriorlythe internal carotid gives off the orbital artery, which runsforwards dorsal to the efferent artery of the spiracular demi-branch but ventral to the ophthalmica magna. In the firstbranchial arch there are now three vessels. The anterior oneis the anterior efferent branchial artery, connected with thedorsal aorta, and at this stage still communicating ventrallywith the ventral aorta. The next one is the afferent branchialartery, leading out of the ventral aorta ; and behind it is theposterior efferent branchial artery. The two efferent branchialarteries are connected half-way up their length by a crosscommissure which runs median to the afferent branchialvessel.

At 45 mm. (Text-fig. 12 d) the efferent hyoid and anteriorNO. 270 X

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TEXT-FIG. 12.

ophma da cc

Diagrammatic representation of the development of the arteriesin H e t e r o d o n t u s . (a) 11 mm., (b) 21 mm., (c) 25 mm.,(d) 45 mm., (e) 70 mm.

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efferent branchial vessels communicate beneath the first gill-slit. The efferent vessels have lost all connexion with theventral aorta, but communicate with a vessel which is the rudi-ment of the hypobranchial artery. The afferent artery to thespiracular demibranch arises from the anterior continuation ofthis hypobranchial artery, not from the ventral aorta. Upto and including this stage the spiracular demibranch has beensupplied by two arteries, one the true mandibular vessel, andthe other a cross commissure from the efferent hyoid artery.The true mandibular vessel lies median to the ramus rnandi-bularis externus facialis, and lateral to the ramus mandibularisinternus facialis, confirming Allis (6). The cross commissureruns median to both these nerves. Since the afferent mandibularvessel arises no longer from the ventral aorta but from thehypobranchial artery, it transmits only blood which hasalready been through another gill. The spiracular demibranchtherefore no longer functions as a true respiratory organ, butis a pseudobranch.

At 70 mm. (Text-fig. 12 e) the true afferent mandibularvessel has disappeared, the pseudobranch being supplied solelyby the cross commissure which becomes known as the afferentpseudobranchial artery. The anterior prolongation of thehypobranchial artery becomes the external carotid.

(ii) E e l a t i o n s of the Arteries.—Before proceeding tothe consideration of the relations of the various arteries itmust be pointed out that with regard to the nomenclaturethere prevails the wildest confusion. Many authors, amongwhom may be mentioned Allis (1, 2, 3, 4), Ayers (10), Carazzi(11), Dohrn (15), Greil (17), Hyrtl (21), Muller (23), Parker (24),and Wright (23), apply the same name to different vessels,and call the same vessel by different names.

In the following table an attempt is made to give the relativeequivalents of terms used by the different authors:Efferent pseudobranchial (Allis)

= Anterior carotid (Parker)= Part of afferent spiracular and carotis interna anterior

(Dohrn).X 2

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Afferent mandibular= Primary afferent pseudobranehial (of A m i a, Allis)= Hyoidea (Wright)= Pseudobranchialis (Parker)= Thyreo-spiracularis and part of afferent spiracular (Dohrn).

Centralis retinae (Dohrn)= Optic (Allis)= Ophthalmica minor (Miiller)= Ophthalmica (Greil).

Ophthalmica magna= Choroidalis (Dohrn)= Orbitalis (Greil).

Orbitalis (Carazzi)= Temporalis (Greil)= Posterior carotid (Parker)= Carotis facialis (Bathke)= External carotid (Allis).

The internal carotid is the anterior prolongation of thedorsal aorta. It runs median to the trabecula cranii and thenrises between them so as to lie dorsal to the trabecular plate(Text-figs. 9 and 11). The external carotid is the anterior pro-longation of the ventral aorta, which may shift its connexionto the hypobranchial artery (Text-fig. 12 e). The efferentpseudobranehial artery represents the dorsal portion of thetrue mandibular vessel. In Heterodontus and all Sela-chians it runs upwards median to the orbital artery and passesdorsal to the trabecula cranii to join the internal carotid.In all other vertebrates it passes ventral to the trabecula.This point will be referred to again below. It passes lateralto the ramus palatinus facialis.

The afferent mandibular vessel, which arose first from theventral aorta and later from the hypobranchial artery to runto the pseudobranch, disappears. It represents the ventralportion of the true mandibular vessel.

The arteria centralis retinae arises from the internal carotidat the point where the latter passes over the optic nerve.

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It accompanies the optic nerve through the optic foramen intothe eyeball (Text-fig. 11).

The ophthalmica magna arises from the efferent pseudo-branchial artery near the junction of the latter with theinternal carotid. It runs laterally into the eyeball, passingdorsal to the orbital artery and anterior to the maxillary andbuccal nerves.

The orbital artery arises from the internal carotid beforethe latter pierces its foramen into the cranial cavity. Theorbital artery pierces a small lateral projection from theparachordal and emerges median and ventral to the facialganglion. It does not, therefore, enter the cranial cavity.It runs forwards along the edge of the pterygoquadrate, passingdorsal to the efferent pseudobranchial and ventral to theophthalmica magna (Text-figs. 11 and 12). It runs anteriorto the ramus palatinus facialis, and at first lies dorsal andlateral to the maxillary and buccal nerves ; further forwardit curves round the latter, lying behind and ventral to them.

The relations of the branchial vessels to nerves and cartilageshave been described in the writer's work on Hete rodontus(13), to which the reader is referred.

The nomenclature of the carotids in these and other formsdeserves- perhaps a little amplification. While the internalcarotid is always the prolongation of the dorsal aorta, whenmost of its blood is derived from the mandibular arch it issometimes called anterior internal carotid; if most is derivedfrom the hyoid or posterior arches, it is a common plus internalcarotid. The so-called common carotid, which is difficult todefine since it is an intermediate portion of the dorsal aortaand has been ascribed different limits along that vessel bydifferent authors, may be regarded as that portion of thedorsal aorta which lies between the junctions of the latter vesselwith the mandibular and hyoid vessels. It is not homologouswith the common carotid of higher vertebrates, which is aventral vessel.

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3. TELEOSTOMES.(a) A m i a c a 1 v a.

(i) The Venae Capitis.—The primitive vena capitismedialis is present in embryos of Ami a 6 mm. long (Test-figs. 13 and 14). It passes median and ventral to all the dorsalnerve-roots and between the auditory sac and the brain. Evenat this early stage, however, it is not the sole vein of the head,

TEXT-FIG. 13.

Reconstruction from the ventral side of Amia calva (6 mm.).

for a small secondary vein is present, which, leaving the medialisjust anterior to the facial nerve, runs lateral to the latterand to the auditory sac, but median to both glossopharyngealand vagus, behind which it rejoins the medialis. The medialisis still the principal vessel at this stage, and it gives off thehypophysial vein median to the trigeminal.

At 7 mm. (Text-fig. 15) the medialis has disappeared behindthe trigeminal, so that from there posteriorly the secondary

VEBTEBBATE HEAD 305

vein is the only one present. It has lapped round the glosso-pharyngeal and now lies external to it, but it is still internalto the vagus. Median to the trigeminal the median's givesoff the hypophysial vein which runs dorsal to the externalrectus muscle, for the latter is already extending backwardsunder the brain.

A reconstruction from the left side of a slightly later stageis shown in Text-fig. 16, with the cartilage omitted.

TEXT-FIG. 14.

Selected sections through Amia (6 mm.) in the region of {a) tri-geminal, (6) slightly behind (a), (c) facial, {d) auditory nerve,(e) auditory vesicle, (/) gloesopharyngeal, (g) vagus, (h) first post-otic myotome.

At 11 mm. the cartilage of the skull is already well developed.The secondary vein is enclosed by the prefacial cartilaginouscommissure, so that it issues from the skull by the facialforamen lying dorsal to the nerve. Here it gives off a smallvein which accompanies the facial through the hyomandibularforamen. Prom the point of its emergence from the skullthere starts a vein whieh, though in contact with the secondaryvein, does not yet communicate with it and ends blindly infront. This vein passes external to both glossopharyngeal

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and vagus, and then comes into contact with the secondaryvein again behind the vagus. Prom its relations to the nerves

TEXT-FIG. 15.

Reconstruction from the ventral side of A mi a (8 mm.

TEXT-FIG. 16.

Reconstruction from the left side of Amia (9 mm.).

this vein is the vena capitis lateralis. Reconstructions of thisstage from the left side in Text-fig. 17, from the inside in

TEX

T-FI

G.

17.—

Rec

onst

ruct

ion

from

the

lef

t si

de o

f A

mia

(1

1 m

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.

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Text-fig. 18, from behind in Text-fig. 19, and from above inText-fig. 21 will make this description clearer. Text-fig. 20is a reconstruction of the cartilage alone for greater ease of

TEXT-FIG. 18.tr

foc

Ipa1

Reconstruction from the inside of Amia (11 mm.).

TEXT-FIG. 19.

Reconstruction from behind of Amia (11 mm.). The skull has beencut transversely in the region of the hyomandibular.

comparison of the relations. Selected sections are reproducedin Text-fig. 22. The secondary vein thus does n o t becomethe lateralis, as it does in Selachians.

Attention is called to the relations which the secondary veinand vena capitis lateralis bear to the hyomandibular. Both pass

VERTEBRATE HEAD 809

ventral and median to this cartilage. The lateralis runs lateralto the hyomandibular muscle, the secondary vein median tothat muscle.

Eeconstructions of a 50 mm. stage are given in Text-figs. 28and 24. There is not much change in the relations of theveins. The lateralis extends forwards beyond the hyomandi-

TEXT-FIQ. 20.

£;&$\ tp

The skull of Amia (11 mm.) from the dorsal side.

bular into the orbital sinus. The median's just anterior tothe point where the hypophysial vein is given off divides intotwo, the oculomotor nerve passing between the branches.

(ii) The Bela t ions of the Arteries.—The internalcarotid passing forward rises in a notch in the trabecula(Text-figs. 20 and 21), which eventually becomes closed to forma foramen. It sends a tiny branch ventral to the trabecularplate, but itself passes dorsal to this cartilage and gives off the

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posterior cerebral artery, the centralis retinae, and the anteriorcerebral. Just before the internal carotid rises over thetrabecula it receives the efferent pseudobranchial artery whichpasses vent ra l to the trabecula. Prom the point of junction

TEXT-FIG. 21.jlCU

Reconstruction of A mi a (11 mm.) from the dorsal side.

the ophthalmica rnagna arises, passing up the median face ofthe trabecula and over it to the eye. At 50 mm. the efferentpseudobranchial no longer runs into the internal carotid, butcontinues straight into the ophthalmica rnagna (Text-fig. 24).The efferent pseudobranchial artery runs external to the ramuspalatinus facialis.

The primary afferent pseudobranchial artery at the 11 mm.

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stage runs down from the pseudobranch and gives off a vesselwhich runs back into the operculum lateral to the hyomandi-bular. It continues downwards, lying between the pterygo-quadrate and the hyomandibular and between the rarni

TEXT-FIG. 22.

£ Selected sections through Ami a (11 mm.) in the regions of (a) tri-geniinal, (6) facial, (c) orbital artery, (d) and (e) hyomandibular,(/) glossopharyngeal, (g) and (h) vagus, (i) first postotic myotome.

externus and internus mandibularis facialis. It passes externalto the symplectic and on to the anterior surface of the cerato-hyal, where it gives off a branch which runs forwards externalto Meckel's cartilage. It then splits into two portions whichreunite and run into the hypobranchial artery (Text-fig. 17).

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TEXT-FIG. 23.

Reconstruction of Amia (50 mm.) from the right side.

TEXT-'FIG. 24.

Reconstruction of Amia (50 mm.) from the front.

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At 50 mm. the primary afferent pseudobranchial artery hasbeen replaced by a secondary afferent artery given off as abranch from the orbital artery (see Allis, 1 and 2). The orbitalartery arises from the internal carotid at about the level of

TEXT-FIG. 25.

7> 9,s/

0

Diagrammatic representation of the development of the arteriesof Amia : (a) 6 mm., (6) 11 mm., wrong interpretation, (c) correctinterpretation.

the facial foramen. It pierces the parachordal and runslateral to the ramus palatinus facialis and ventral to themedialis vein and the trigeminal nerve (Text-fig. 18), withwhich it emerges from the skull (Text-fig. 24).

The afferent hyoid artery arises from the ventral aorta,as do the afferent branchials. The efferent hyoid artery runs

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ventral to the lateralis and secondary veins and joins the dorsalaorta close to the orbital artery.

The hypobranchial arteries join in the middle line ventral

TEXT-FIG. 26.

Reconstruction of the ventral arteries of A mi a (11 mm.) fromthe dorsal side.

to the afferent hyoid artery and connect with a mandibularsinus (Text-fig. 26). Behind the afferent hyoid artery theyjoin dorsal to the ventral aorta, and then run back ventral-to the afferent branchial vessels. Posteriorly the hypobranchial

VERTEBRATE HEAD 315

arteries connect with the ductus cuvieri on each side. Thisconnecting vessel (which is functional and contains bloodcorpuscles) is shown in Text-figs. 17 and 26. Of the functionsof this connexion it is hard to be certain. The pressure of theblood must be higher in the hypobranchial arteries than inthe ductus cuvieri, so that the direction of flow would be tothe latter vessel.

The result of this would be a supply to the heart of bloodmore oxygenated than would otherwise be the case, possiblyto meet the respiratory requirements of the heart itself, forwith branchial respiration the heart is entirely venous. Theconnexion could not be traced at 50 mm.

(iii) D e v e l o p m e n t of t h e Arter ies .—The efferentpseudobranchial artery lies anterior to the spiracle, andtherefore certainly represents the dorsal portion of the originalmandibular vessel. Ventrally the spiracular slit is not open,but a deep depression shows its position, and the primaryafferent pseudobranchial artery, running along the ceratohyal,lies behind this depression (Text-fig. 26). The question there-fore presents itself, is the afferent pseudobranchial artery theventral portion of the mandibular vessel which has slipped back,as Allis (1) suggests (represented diagrammatically in Text-fig. 25c), there being one hyoid vessel (the afferent and efferenthyoid vessels being a continuation the one of the other) ?Or is the afferent pseudobranchial vessel a cross commissurefrom a hyoid vessel as in H e t e r o d o n t u s , the hyoid vesselhaving split ventrally into two (represented diagrammaticallyin Text-fig. 25 b) ?

In embryos 6 mm. long there is present a single vessel ineach of the first two visceral arches, the mandibular and thehyoid (Text-fig. 25 a). Beneath the spiracle these pre- and post-spiracular vessels are connected by a cross commissure. Atthe 11 mm. stage the hyoid vessel is interrupted, the dorsalportion being the efferent hyoid artery. If the second alterna-tive be true, then the ventral portion would be split into two,as in the branchial arches, one connecting with the ventralaorta, the other with the hypobranchial artery. The presence

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in the hyoid arch of the afferent hyoid artery does not precludethe possibility of the afferent pseudobranchial artery beingderived from the primitive post-spiracular vessel, and con-nected with the mandibular vessel by the cross commissureof the previous stage.

But this rests on the assumption that a break has occurredin the mandibular vessel and that the ventral portion has eitherdisappeared or lost connexion with the dorsal portion. Acareful search through a series of embryos of 7, 7-5, 8, 8-5, and

TEXT-FIG. 27.Orb A

T6 Ppq NC

Reconstruction of Salmo t ru t t a (20 mm.) from the left side.

9 mm. has not revealed any such interruption : the mandibularvessel is continuous the whole time. The cross commissureloses connexion with the post-spiracular vessel and extendsback into the operculum. The primary afferent and efferentpseudobranchial arteries therefore represent the original mandi-bular vessel. The relations of the rami internus and externusfacialis to this vessel (Text-fig. 25 c) are similar to those inH e t e r o d o n t us for the mandibular vessel.

(6) S a l m o t r u t t a .

(i) T h e V e n a e Capi t i s .—In 20mm. trout embryos theprimitive vena capitis medialis has already long since dis-appeared, except in the region of the trigeminal where it gives

VERTEBRATE HEAD 817

off the hypophysial vein. As in A mi a, the secondary veinis enclosed by the prefacial cartilaginous commissure, andit emerges through the facial foramen, but ventral to the rarnushyomandibularis facialis, not dorsal and external to it as in

TEXT-FIG. 28.

Reconstruction of Sal mo (20 mm.) from the dorsal side.

Ami a. Thereafter it continues backwards outside the skulland internal to the glossopharyngeal and the vagus. Thevena capitis lateralis starts at the point of exit of the secondaryvein from the skull (and in contact with it) and courses back-external to the facial, glossopharyngeal, and vagus, to re-establish connexion with the secondary vein behind the latternerve (Text-figs. 27, 28, and 29).

Y 2

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Both the lateralis and the secondary vein pass internal tothe hyomandibular and separated from one another by thehyomandibular muscle, as in Ami a. The lateralis is similar

TEXT-FIG. 29.

Selected sections through Sal mo (20 mm.) in the regions of(a) trigeminal, (6) orbital artery, (c) facial, (d) and (e) hyomandi-bular, (/) glossopharyngeal, (g) vagus, (h) first postotic myotome.

to that vessel in A m i a, but the secondary vein differs fronithat form in that it passes internal instead of external tothe facial.

The hypophysial vein runs dorsal to the external rectusmuscle, which extends back in the myodome. The oculomotorruns in between two branches of the medialis vein.

(ii) The Arteries.—The efferent pseudobranchial artery

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runs under the trabecula, as in Ami a, to join the internalcarotid. The ophthalmica magna runs over the trabeculaout to the eyeball. Allis (7) quotes Dohrn as saying that atan early stage in the trout the ophthalmica magna runs underthe trabecula. I have examined embryos 6 mm. 8 mm.,10 mm., 20 mm., and 50 mm. long, but have not found this.

The orbital artery leaves the internal carotid and piercesthe parachordal beneath the facial nerve. It runs forward

TBXT-FIO. 30.

. - tr

hmF

Reconstruction of Cottus bubalis (11 clays) from the front.

along the upper face of the cartilage ventral to the externalrectus muscle and across the orbit.

The arteries in the trout are on the whole very similar tothose of Ami a.

(c) Cottus bubal is .The Venae Capitis.—In the Te leos tomes so far

examined all the veins have been found to lie median to thehyomandibular ; the medialis of course, the secondary vein,and the lateralis were all covered over by the hyomandibular.In Cottus bubalis (11 days, Text-figs. 30 and 81) theconditions are fairly similar to those obtaining in A m i a andthe trout. The medialis disappears behind the trigeminal

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owing to the development of the auditory capsule, and isreplaced by a secondary vein and the lateralis, both insidethe hyoniandibular. But lateral to the hyoniandibular there

TEXT-FIG. 31.

Selected sections through C o t t u s (11 days) iu the regions of(a) trigeminal, (6) and (c) hyoniandibular, (d) glossopharyngeal.

is a small vein which, running into the orbital sinuses in front,joins the other two veins behind, in the region of the glosso-pharyngeal.

4. DIPNOI.

C e r a t o d u s f o r s t e r i .

(i) The V e n a e Cap i t is.—In a 18 mm. embryo ofC e r a t o d u s (Text-figs. 32, 33, 34, and 35) the cartilage is

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TEXT-ITO. 32.

Reconstruction of Geratodus forsteri (13 mm.) from theleft side. From a wax model.

TEXT-FIG. 33.

Reconstruction of Ceratodus (13 ram.) from the inside.

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already well developed. The figures reproduced here are froma wax reconstruction.

Anteriorly the head veins start from the orbital sinusesand run back. They divide into two, one branch passing

TEXT-FIG. 34.

gl---'

Reconstruction of C e r a t o d u s (13 mm.) from the dorsal side.The roof of the auditory capsule and of the foramen prootioumhave been removed on the left side.

lateral to and the other median to the processus ascendens.The branch enclosed by the processus ascendens lies beneath theranius ophthalmicus profundus, and gives off the hypophysialvein medially.

Behind the processus ascendens both branches of the veinunite and pass back median to the trigeminal and facial nerves

VERTEBRATE HEAD 323

and to the processus oticus, dorsal to the processus palato-basalis.

The hyomandibular element is very small at this stage(Text-fig. 42 a), and the vein passes external to it and internal

TEXT-FIG. 35.

Selected sections through C e r a t o d u s (13 mm.) in the regionsof (a) vagus, (b) glossopharyngeal, (c) facial, (d) palatobasal process,(e) processus ascendens, (/) orbital sinus.

to the hyomandibular muscle. Lateral to this muscle there isa cord of dense tissue of which Greil (17) says :

' Ein verdickter Zellenstrang zieht von der dorsalen Kantedes Keratohyals zum seitlichen Eand des Labyrinthknorpelsund stellt wohl die Anfange kleiner Vorknorpelher.de dm-'(p. 1399).

The presence of these structures lateral to the vein is of interestand will be referred to again. The vein continues backwardsexternal to the glossopharyngeal, and it laps round both sidesof the vagus. The vein at this stage is therefore lateralis asfar forward as the facial, then medialis.

(ii) T h e Arter ies .—The efferent pseudobranchial vessel

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(mandibular) runs up between the pterygoquadrate and theceratohyal, and joins the internal carotid, passing under thetrabecula. The ophthalmica magna arises somewhat furtherforward from the internal carotid and passes out to the eyeover the carotid, piercing a small foramen in the side Avail ofthe skull. Still further forward the arteria centralis retinaeleaves the internal carotid and accompanies the optic nerveinto the eyeball.

The orbital artery leaves the internal carotid behind thejunction between the latter and the efferent pseudobranchial.It passes ventral and lateral to the vein and to the facial,and median to the processus oticus. It then curves up slightlyso as to lie lateral to the ganglia and between the ramusbuccalis and the superficial ophthalmic, and continues into theorbit.

5. CYCLOSTOMES.Petromyzon.

(i) The Venae Gapitis.—Little or nothing remains to bediscovered with regard to the blood-vessels of lampreys afterCori's (12) careful work and Hatta's (18) monumental paper.The reconstructions given here in Text-figs. 36, 37, and 38are not intended to supplement or contradict the works ofthese authors, but merely to give the relations of only themost important vessels for comparison with other forms.

Text-figs. 36 and 37 are from an 8 mm. embryo of Pe t ro-myzon planer i , and the most striking thing about it isthat the principal vein is the lateralis. It passes back over theeye, trigeminal and facial nerves, auditory sac and glosso-pharyngeal nerve, and branchial branches of the vagus. Itlies median to the ramus communicans between the facial andvagus nerves.

Behind the vagus the vein descends so as to lie ventral tothe lateralis ganglion of the vagus, and it then joins two otherveins. One of these is the vena capitis medialis, which runsforwards median to the vagus and glossopharyngeal. Butit does not reach as far forward as the facial, for it curves

VERTEBRATE HEAD 325

TEXT-FIG. 36.

Keconstruction of P e t r o m y z o n p l a n e r i (8 mm.) from theleft side.

TEXT-FIG. 37.

Selected sections through P e t r o m y z o n (8 mm.) in the regionsof (a) in front of eye, (6) eye, (c) trigeminal, [d) facial, (e) glosso-pharyngeal, (/) vagus, (g) first postotic myotome.

326 G. R. DE BEER

down in the hyoid arch, i. e. the anterior wall of the firstbranchial basket, between that basket and the velar recess.In so doing the vein passes down lateral to the dorsal aorta,but in the arch it lies median to the hyoid artery.

The other vein is known as the facial. It starts far forwardin the lateral body-wall and connects with the mandibularvessel from the ventral aorta. It passes back lateral to allother veins, arteries, or cartilages. Behind the vagus it con-nects with the vein formed by the fused medialis and lateralis

TEXT-FIG. 38.

Reconstruction of Ammocoete of Petromyzon fluviatilisfrom the left side.

by a connecting vein which passes median to the ramus bran-chialis vagi. Behind this the facial vein continues its courseconnected with the lateralis in the intervals between thebranchial baskets.

In an Ammocoete larva of Petromyzon f luviat i l isthe conditions are not dissimilar (Text-fig. 38). The lateralisvein connects with a dorsal occipital sinus and passes backmedian to the ramus communicans and lateral to the glosso-pharyngeal and vagus. Behind the ramus branchialis vagithere emerge two conjoined ventral nerve-roots. The moreanterior of these left the brain level with the glossopharyngeal,the other level with the vagus. These two roots run back

VERTEBBATE HEAD 327

within the connective tissue investment to the neural tube,and emerge behind the branchialis branch of the vagus. Theythen curve outwards and forwards, passing lateral to the lateralisvein, and are distributed to the first two postotic myotoraes(segments numbers four and five). The existence of thesenerves and their relations to the veins are insisted upon herebecause it has often been stated that the anterior postoticmyotomes of Petromyzon lack nerve-roots. The nervesdescribed here appear to be the same as those which Johnston(22) describes in Petromyzon dorsatus , bub which hesays are absent in other species. They can readily be foundin serial transverse sections of Ammocoetes without neuro-logical technique.

Anteriorly the facial vein has for a short distance becomesplit into two. It connects down the mandibular (velar)arch, no longer with the ventral aorta but with a vessel towhich the latter has given rise by splitting. It is from thisvessel, which lies lateral to the cartilages of the branchialbasket, that the vena capitis median's arises, passing up thehyoid arch.

The venous system of P e t r o m y z on is peculiar, and canwith difficulty only be compared with that of other forms.The existence of the facial veins in the head is a peculiarfeature. They are possibly homologous with the lateral veinsof fish, and persist in the head because there is no maxillo-mandibular articulation to crowd them out. The early appear-ance of the lateralis is perhaps the most striking feature.

(ii) The Arteries.—At the 8mm. stage the arterial andvenous sytems are not clearly separated, for the facial veinconnects with the ventral aorta by the mandibular vessel.

In the Ammocoete the mandibular vessel has split into two.One of the so-formed elements connects with the facial veinas described above. The other connects with the internalcarotid, passing ventral to the trabecula (anterior parachordalsof Sewertzoff, 30). The ventral connexion between this mandi-bular vessel and the ventral aorta is difficult to make out.Near the point where the mandibular vessel joins the internal

328 G. E. DB BEER

carotid a small artery is given off which passes up median tothe cartilage and up towards the brain. This is Cori's carotiscerebralis and Demme's (14 a) internal carotid. The mainbranch of the internal carotid continues forwards (Demme'sexternal carotid) beneath the cartilage and rises up laterallyto them. If these cartilages are the true trabeculae, then thesmall artery passing up median to them is the true internalcarotid, and the so-called internal carotid is something else.If on the other hand the cartilages are anterior parachordals,as suggested by Sewertzoff (30), then the main artery can stillhardly be the true internal carotid, since it does not enter thecranial cavity.

6. CONSTANT RELATIONS.

In the forms which have been examined some structureshave been found to bear constant relations to the neighbouringstructures. Among these is the vena capitis medialis (exceptfor the curious variation in cyclostomes). It passes medianto the dorsal nerve-roots and to the auditory sac. Median tothe trigeminal it gives off the hypophysial vein, which entersthe skull anterior to the pila antotica and passes dorsal tothe internal carotid and the external rectus muscle, when itextends back into a myodome.

The vena capitis lateralis bears constant relations to thenerves (lying outside them) but not to the cartilages, aswill be shown below. It is, however, always lateral to theauditory sac.

With regard to the arteries, the ophthalmica magna passesdorsal to the trabecula, as does the centralis retinae. Theorbital artery emerges from the internal carotid behind thepila antotica, either through its own foramen or with thetrigeminal nerve. The efferent pseudobranchial artery (mandi-bular vessel) passes external to the ramus palatinus facialison its way to the internal carotid, although its relations withthe trabeculae vary. These constant relations are diagram-matically represented in Text-fig. 39.

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7. INCONSTANT RELATIONS.

(a) The Mandibular Vessel.—It has already beennoticed that in Selachians the efferent pseudobranchial vesseljoins the internal carotid by passing over the trabecula, whereasin all the other groups it passes under the trabecula. This isa discrepancy which needs explanation. The two types areshown in Text-fig. 40 c and a. Since the Selachians are the

vcfn

Diagram of the constant relations of veins and arteries to thecartilages.

only forms (Allis, 7 and 9, mentions Amiurus as being ofthis type though a Teleost; in this case, however, thecondition is secondary) in which the vessel goes over thetrabecula, and in all other craniates it passes under thatcartilage with constant regularity, it is probable that theSelachian condition is secondary. How, then, can it have beenderived from the other type ? It might be supposed that thegreat cranial flexure characteristic of Selachians and theangle which the trabeculae make with the long axis may beresponsible for the pseudobranchial artery slipping in between

330 G. R. DE BEER

the trabeculae and parachordals before they fuse. ButA m n i o t e s also have the cranial flexure, and the relations inthem are not the same as in Selachians. Besides, at the firstappearance of the trabeculae the artery is already dorsal to thecartilage.

Additional evidence against this view is found when theophthalmica magna is considered. In Selachians this arterycomes off from the efferent pseudobranchial, some distanceaway from the internal carotid; in the other forms it comesoff close to the internal carotid. This looks as if the vesselbetween the internal carotid and the beginning of the ophthal-mica magna in Selachians, and the base of the ophthalmica

TEXT-FIG. 40.

Relations of the trabecula cranii to the arteries in (a) non-Selachians,(c) Selachians, (6) hypothetical intermediate condition assumingthat the cartilage remained unchanged.

magna in the other groups, were the same vessel. The efferentpseudobranchial artery would have fused with the ophthalmicamagna lateral to the trabecula, and the original junction of theefferent pseudobranchial with the internal carotid would havebeen lost, in producing the Selachian type (Text-fig. 406).To account for the difference there must be a change in therelations of either the vessels or the cartilage. Recently Allis(9), while maintaining that the vessels remain unchanged, hasadvanced a most ingenious explanation. Starting from theconditions in a branchial arch he notes that in the sharks theefferent vessel joins the aorta over and in front of the pharyngo-branchial, whereas in T e l e o s t o m e s it passes behind thepharyngobranchial (Text-fig. 41 a and b). Now turning tothe polar cartilage (see Pehrson, 25, van Wijhe, 32), in sharks

VERTEBRATE HEAD 331

the efferent pseudobranchial artery passes over and in frontof it to join the internal carotid (Text-fig. 41c), in Teleo-

(a) Relations of the branchial cartilages to the arteries in He te ro -don tus (Selachian). The dotted lines represent the positionswhich the trabecula and parachordal would occupy if the figurewere dealing with the mandibular arch and the polar cartilagewere the pharyngomandibular.

(6) Relations of the branchial cartilage to the arteries in Amia(Teleostome).

(c) Relations of the trabecula, polar cartilage, parachordal, andarteries in Squalus (30 mm.) (as interpreted, by van Wijhe).

(d) Relations of the trabecular, polar cartilage, parachordal, andarteries in Amia (12 mm.) (as interpreted by Pehrson).

(e) Hypothetical interpretation of the mandibular arch in theTeleostome if the arch be of the sigma type.

stomes it passes beneath. If the polar cartilage representsthe pharyngeal element of the mandibular arch, then its

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relations to the mandibular vessel are directly comparablewith the relations of the pharyngobranchial to the branchialvessel in the two respective groups. If now the trabeculaattaches itself to the polar cartilage from in front, forminga single rod with it, that rod will lie under the mandibularvessel in sharks and over it in Teleostomes, which willgive the required relations. In other words, Allis regards thechange as brought about by the mandibular vessel passingbetween the polar cartilage and the trabecula, but that it isreally passive to the movements of the cartilages. In Text-fig. 41 a and b the dotted lines indicate the positions whichthe trabecula and parachordal would occupy assuming thepolar cartilage to correspond to the pharyngobranchial. Allisattributes the difference as ' evidently due to the fact thatthe internal carotid (lateral dorsal aorta) has not here (in theTeleostome) been crowded inwards to the median line bythe fusion of the pharyngomandibulars (polar cartilages) ofopposite sides, and that the latter cartilages have not fusedwith the axial skeleton until after they and the pharyngo-premandibulars (trabeculae) have both acquired a positionin the horizontal plane of the parachordals. Because of thisthe ventral end of the pharyngomandibular, in swingingupwards , has passed posterodorsal to the efferent arteryof its arch '.

It is comprehensible that the fusion of the polar cartilagesof opposite sides and the consequent approximation of theinternal carotids in Selachians would result in pulling theefferent mandibular vessel inwards and backwards so that it' crosses the anterior edge of the polar cartilage definitelydorsal to its ventral end '. Thus when the trabecula fuseswith this ventral end of the polar cartilage the bar thus formedunderlies the mandibular vessel. But I do not see how theventral end of the polar cartilage in swinging upward can passposterodorsal to the efferent artery of its arch in Teleo-stomes. If the mandibular arch must be of the sigma type.i.e. with the pharyngeal element directed backwards, thenecessary relations would be obtained if the ventral end of

VERTEBRATE HEAD 333

the pharyngomandibular were to swing forwards and outwards.But if this were so (Text-fig. 41 e) then the mandibular vesselwould lie anterior to the epimandibular or palatoquadrate.The fact that it lies posterior to the processus palatobasaliswould have to be explained by its slipping through backwardsbetween the pharyngeal and epal elements of the mandibulararch.

If on the other hand the mandibular arch in T e 1 e o s t o m e swere of the V type, and the pharyngomandibular acquiresconnexion with the trabecula and parachordal (Text-fig. 41 b),no rotation would be required.

Beyond analogous cases of looping there is no direct evidencefor the change being brought about by the looping of themandibular vessel, and in the present state of knowledge itappears to me that Allis's view or sDme modification of it isthe more probable explanation. He makes a strong case forthe polar cartilage being a pharyngomandibular. Whether thetrabecula is a pharyngopremandibular or not is open toquestion, but this question does not really affect the problemat issue here.

(b) The Hyomandibular.—In Selachians (Text-fig. 42b)the hyomandibular articulates with the auditory capsule half-way between the notochord and the lateral edge of the capsule.External to it, originating from the edge of the capsule, is thehyomandibular muscle, and between the latter and the hyo-mandibular passes the lateralis vein.

InTeleostomes (Text-fig. 42 c) the hyomandibular articu-lates with the auditory capsule right at its lateral edge. Medianto it is the hyomandibular muscle and the two veins, secondaryand lateralis.

The hyomandibular therefore bears different relations bothto the veins and to the muscles in the two groups. Not onlythis, but its relations to the ramus hyomandibularis facialisare different, for in Selachians this nerve passes back dorsaland external to the hyomandibular (Text-fig. 9), Avhereas inmost Teleostomes it pierces the hyomandibular (Text-figs. 19, 21, 22, 23, 24, 27, 28, 29, and 31).

334 0. B. DB BEER

The change of relations is accompanied by a change offunction in the two groups. In Selachians the hyomandibularis either the simple skeletal element of the dorsal portion ofthe hyoid arch, or it assists in suspending the pterygoquadrate.In Teleostomes the hyomandibular, firmly united with theopercular, is responsible for supporting the operculum. Theoperculum is lateral to the remaining visceral arches, so thatit is not surprising that the hyomandibular in these formsshould arise from the lateral edge of the auditory capsule.

The possibility at once suggests itself that the hyomandi-bulars in the two groups are not really the same element(Allis and Woskoboinikoff, quoted by Schmalhausen, 29).Pollard (25 a) also came to this conclusion, but as he regardsthe spiracle and facial nerve as variable with regard to thecartilage, his views cannot be accepted.

It is a familiar fact that the mandibular arch frequentlyhas two or more articulations with the neurocranium (Text-fig. 42 e). Of these the processus palatobasalis is the originaltop of the arch. It lies median to the vein and to the nerves.The other, processus oticus, articulates with the skull in amore lateral position ; it lies external to the vein and to thenerves. In origin the processus oticus is probably, as Allissuggests, a modified branchial ray. May not the hyoid archalso have two articulations ? The hyomandibular of Selachians,articulating with the skull in a mesial position, median to theveins and nerves, is comparable with the proeessus palato-basalis of the mandibular arch and the pharyngobranchialsof the other visceral arches, and represents the true primitiveposition of articulation of the hyoid arch. Whether it repre-sents the pharyngo- or epi-elements is another question.The hyomandibular of Teleostomes, however, wouldrepresent a ' processus oticus hyomandibularis '.

In the mandibular arch either of the two processes may bereduced. In Heptanchus there is a processus oticus anda palatobasalis; in Heterodontus there is apparentlya palatobasalis without an oticus.

The schematic hyomandibular would then be represented in

VERTEBRATE HEAD 835

Text-fig. 42 d. That the two processes are not found simul-taneously must be due to the fact that the double articulationwould not allow sufficient mobility for the operculum. InCeratodus , however, it would appear that this idealschematic condition is realized (Text-fig. 42 a). At the 13 mm.

TEXT-FIG. 42.

hmms

Relations of the hyomandibular cartilage in (a) C e r a t o d u s ,(b) S c y l l i u m , (c) A m i a , (d) schematic hyomandibular withotic process to compare with (e) palatobasal and otic articula-tions of the mandibular arch.

stage there is a very small hyomandibular articulating half-way between the notochord and the edge of the auditorycapsule, median to the vein and the muscle. This cartilageis the oto-quadrate cartilage of Edgeworth, the hyomandibularof Krawetz (22 a), Eidewood (27 a). Huxley's (20 a) and vanWijhe's (32 a) hyomandibular may be the same element, butI am not sure. Lateral to the muscle is the strand of radi-

386 G. R. DE BEER

nientary procartilage described by Greil in the passage alreadyquoted. This latter structure would be the processus oticushyomandibularis, while the hyomandibular figured in Text-fig. 42 a would represent the true original cartilage of that name.The presence of a small extra vessel outside the hyomandibularin Cottus cannot be of much importance.

Edgeworth (16) considers the development of the hyomandi-bular in C e r a t o d u s. What I have called the hyomandibularhe terms the' oto-quadrate cartilage '. Lateral to it he describesan interhyal, which is obviously the structure produced bythe ' Zellenstrang ' of Greil (cited above). He does not, how-ever, give the relations which it bears to the vena capitis,although from his statement that it lies between the la te ra ledge of the auditory capsule and the hyosuspensorial ligamentit is probable that it is lateral to the vein.

Since the observations set forth in this paper were madeI have come across Schmalhausen's (29) work. He definitelyshows the lateral hyomandibular cartilage in Ceratodus(lateral to the vein) at the same time as the median one (myhyomandibular). He also states that the same conditionsobtain in embryonic stages of A c i p e n s e r.

He agrees with me in tracing the Selachian and Teleo-s t o m e from an intermediate form, but it does not appear tome that his interpretation of the changes is the correct one.Starting from the assumption that the hyomandibular so calledis really the epihyal in the different fish, he supposes that thedifferent types may have been brought about by a change inthe position of articulation of the epihyal with the skull.It is probable that the hyomandibular of Selachians is theepihyal (see Sewertzoff, 31), but if this same structure can beregarded as capable of developing a new articulation withaltogether different relations to the surrounding structures,the criteria of homology must be somewhat strained. It isto me more probable that the primitive position of articulationshould be occupied by the true dorsal portion of the hyoidarch, i. e. pharyngohyal or epihyal or both, and that, as Allissuggests (see below), the lateral articulation of Ceratodus

VERTEBRATE HEAD 337

(and T e 1 e o s t o in e s) represents a modified gill-ray, compar-able to the processus oticus of the palatoquadrate.

There remains the problem of the relations of the hyo-mandibular to the ramus hyomandibularis facialis. In theSelachian it passes over and external to the cartilage, as do theother branchial nerves to the branchial cartilages in all fish.This is the primitive condition.

In the Teleostome when the nerve pierces the so-calledhyomandibular it is improbable that it has acquired thisposition by eating through the cartilage. No intermediatestages are known either in phylogeny or in development.Allis's (5 and 8) suggestion to account for this is the probableexplanation, and it fits in with the remarks already made hereabout the hyomandibular. He regards the Teleostomehyomandibular as having two heads. The anterior of theseis an interarcual cartilage lying over and outside and in frontof the nerve. The posterior is a modified hyal ray or dorsalextra-hyal cartilage lying behind the nerve. The two havebecome fused together and enclose the nerve between them.Swinnerton (31 a) describes the development of these two headsin Gasterosteus.

In Polypterus the ramus hyomandibularis, which nor-mally divides into mandibularis and hyoideus half-way downthe hyoid arch, divides before it has passed the hyomandibular(so called), the mandibular branch passing in front of it, thehyoideus behind (Allis, 8 a). This would result if a hyal ray,originally below the fork of these nerves, were to extenddorsally and articulate with the auditory capsule.

In the adult Acipenser there is only one head to theso-called hyomandibular, the one corresponding to the anteriorone of other Teleostomes, so that in this form the ramushyomandibularis facialis lies behind and beneath the cartilage.

8. CONCLUSIONS.

In this study an attempt has been made to trace the homo-logies between certain arteries, veins, and cartilages in thefish. In discussing relations it is necessary to start from some

388 G. E. DE BEER

fixed point to which reference can be made. When relationsdiffer in different forms, either one or the other of the twoelements in question may have been responsible for the change,and it becomes necessary to inquire which. In the case ofthe efferent pseudobranchial artery and the trabecula it isnot yet certain which has moved, although the probability isthat it is the cartilage. With regard to the hyomandibular,the changed relations involving not only the veins but also themuscle and nerve render the problem easier to solve by imagin-ing that it was the cartilage that changed.

Detailed comparisons between the vessels of different formsnearly always break down. For instance, the orbital arterymay arise from the internal carotid or from the hyoid artery,or even from the first efferent branchial (Ce ra todus ) . Somesuch variation may account for the fact that it is anterior tothe palatine nerve in Selachians and posterior to it in T e 1 e o-s t o m e s . The ophthalmica magna may arise from the internalcarotid or from the efferent pseudobranchial. Blood-vesselsare therefore capable of changing their relations and connexions,as has been shown by Broman (10 a).

Bather than its detailed connexions, it is the general positionand relations of a vessel which are the best clue to its honio-logies, which may be only partial. This is at one with Eoux's(28) discovery that the rudiments of the blood-vessels are self-differentiating, inherited, and therefore susceptible to analysisby homology ; whereas their later development is conditionedby dependent differentiation. So the orbital artery, which isprobably derived from the dorsal commissural vessel joiningthe efferent branchials, may, if more of the latter vessel persists,join the first efferent branchial. If less persists, it will join theefferent hyoid artery, or even the internal carotid itself.

Changes in relations are not always effected in the sameway. The transition from median's to lateralis in Selachiansis from one vein direct to the other, without the interventionof a separate and distinct secondary vein as in T e l e o s t o m e s .The relations of the secondary vein itself differ in Ami aand Sal mo.

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9. SUMMARY.

In a paper of this nature it is difficult to epitomize theresults any more briefly than has been clone in the actualtext. Only a few of the more important conclusions will besummarized.

1. The vena capitis median's is the primitive head vein.It is replaced by the vena capitis lateralis by different methodsin the different groups.

2. The relations of the efferent pseudobranchial artery to thetrabecula cranii differ in Selachians and the remaining Craniates.This may be explained by vessel-looping in the Selachian orby the different relations of the polar cartilage.

3. The relations of the hyomandibular to the veins, hyo-mandibular muscle, aad facial nerve in the different groupssuggest that the hyomandibular (i) of Selachians is a truehyomandibular, (ii) of T e l e o s t s and A m i a a compound ofan interarcual cartilage and a processus oticus hyomandibularis,the latter being a modified extra-hyal.

4. The schematic condition of the true hyomandibular withthe processus oticus hyomandibularis is shadowed in C e r a-t o d u s , embryonic stages.

5. A certain number of constant relations exist, as set forthon p. 328.

10. LIST OF LITERATURE CITED.

1. Allis, E. P.—" Muscles and nerves in Amia calva ", ' Journ. Morph.',12, 1897.

2. "Pseudobranchial circulation in Amia calva", ' Zool. Jahrb.Abt. Anat.', 14, 1901.

3. "Pseudobranchial and carotid arteries in Gnathostonie fish",ibid., 27, 1909.

4. ——" Pseudobranchial and carotid arteries in Esox, &c.", ' Anat.Anz.', 41, 1912.

5. " The homologies of the hyomandibular of Gnathostorne fish ",' Journ. Morph.', 26, 1915.

6. " The so-called Mandibular artery, &c", ibid., 27, 1916.7. "The Myodome and trigejnino-facialis chamber", ibid., 32,

1919.

340 G. R. DE BEER

8. Allis, E. P.—" On the origin of the hyomandibular ", ' Anat. Eec.', 15,1918.

8 a. "The cranial anatomy of Polypterus", ' Journ. Anat.', 57,1922.

9. " Are the polar and trabecular cartilages . . . &c. ? " ibid., 58,1923.

10. Ayers, H.—" Morphology of the Carotids ", ' Bull. Mus. Comp. Zool.Harvard', 17, 1889.

10a. Broman, I.—" Uber die Entwicklung, Wanderung und Variation derBauchaortenzweige ", ' Anat. Hefte', 2. Abt., 16, 1906.

11. Carazzi, D.—" Sul systema arterioso di Selache maxima ", 'Anat.Aiiz.', 26, 1905.

12. Cori, C. I.—" Das Blutgefasssysteni des jungen Ammocoetes ", ' Arb.Zool. Inst. Wien ', 16, 1906.

13. de Beer, G. R.—"Contributions to the development of the head ofHeterodontus ", ' Quart. Journ. Micr. Sci.', 68, 1924.

14. " The prootic Somites of Heterodontus and of Amia ", ibid., 68,1924.

14 a. Demine, R.—' Das arterielle Gefasssystem von Acipenser ruthenus ',Wien, 1860.

15. Dohrn, A.—" Studien zur Urgeschichte des Wirbeltierkorpers",' Mitt. Zool. Stat. Neapel', 9, 1890.

16. Edgeworth, F. H.—" Note on the development of the quadrate andepihyal", ' Quart. Journ. Micr. Sci.', 67, 1923.

17. Greil, A.—" Entwicklungsgeschichte von Ceratodus ", Semon's' Forschungsreise ', Jena, Denkschr. 4, 1913.

18. Hatta, A.—" Entwicklung des Blutgefasssystems des Neunauges ",' Zool. Jahrb., Abt. Anat.', 44, 1923.

19. Hochstetter, F.—" Venensystem der Amphibien und Fische " , ' Morph.Jahrb.', 13, 1888.

20. "Die Entwicklung des Blutgefasssystems ", Hertvig's ' Hand-buch der Entwicklungslehre ', 6, Jena, 1906.

20a. Huxley, T. H.—" Contributions to morphology. Icthyopsida I ",' Proc. Zool. Soc.', 1878.

21. Hyrtl, J.—" Die Kopfarterie der Haifische ", ' Denkschr. Wien.Akad.', 32, 1872.

22. Johnston, J. B.—"The cranial nerve components of Petrornyzon ",' Morph. Jahrb.', 34, 1905.

22a. Rrawetz, L.—" Entwicklung des Knorpelsohadels von Ceratodus >:,' Bull. Soc Imp. Nat. Moscou ', 24, 1911.

23. Miiller, J.—" Vergleichende Anatomie der Myxinoidea ", ' Abh. K.Akad. Wiss. Berlin', 1839.

24. Packer, T. J.—"On the blood-vessels of Mustelus", 'Phil. Trans.Roy. Soc.', 1886.

VERTEBRATE HEAD 841

25. Pehrson, T.—"Some points in the development of Teleostoniianfishes",' Acta Zool.', 3, 1922.

25 a. Pollard, H. B.—" The suspension of the jaws in fish ", ' Anat. Anz.',10, 1895.

26. Rabl, P.—" tlber die Entwicklung des Venensystems der Selachier ",' Festschr. f. Leuckart', 1892.

27. Raffaele, D. F.—" Ricerche sullo sviluppo del sistema vascularenei Selacei ", ' Mitt. Zool. Stat. Neapel', 10, 1892.

27«. Ridewood, W. G.—"On the hyoid arch in Ceratodus ", ' Proc.Zool. Soc.', 1894.

28. Roux, W.—' Gesammelte Abhandlungen fiber Entwicklungsmechanikder Organismen ', 1895.

29. Schmalhausen, J. J.—" Der Suspensorialapparat der Ksche ", ' Anat.Anz.', 56, 1923.

30. Sewertzoff, A. N.—" Recherches sur l'origine des vertebres in-ferieurs " , ' Arch. Russ.', 1916.

31. " Die Morphologie des Visceralapparates der Elasraobranchien ",' Anat. Anz.', 56, 1923.

31 a. Swinnerton, H. H.—" The Morphology of the Teleostean headskeleton ", ' Quart. Journ. Micr. Sci.', 45, 1902.

32. Van Wijhe, J.—" Friihe Entwicklungsstadien des Kopf- und Rumpf-skeletts von Acanthias " , ' Bijdr. t. d. Dierkunde', 22, 1922.

32a. "Uber das Visceralskelett und die Nerven des Kopfes derGanoiden und von Ceratodus ", ' Nied. Arch. Zool.', 5, 1882.

33. Wright, J. R.—" On the hyomandibular clefts and Pseudobranchs ofLepidosteus and Amia ", ' Journ. Anat. and Phys.', 19, 1885.