VII.- -THE FUNCTION OF THE PERONEUS TERTIUS

MUSCLE.

By W. Ramsay Smith, B.Sc., Demonstrator of Anatomy, Edinburgh School of Medicine, Minto House.

Recently, in the course of studying the actions of the muscles of the lower extremity with the view of determining what muscular movements take place in walking, I was fortunate to meet with some clinical cases worth recording. In this note I shall confine my remarks to two cases illustrating the function of one muscle, the peroneus tertius. The first case was one in which the peroneus tertius of the left

side was subjected to a continued strain by the patient sitting for an hour or two in a cramped position, with the heel on the ground, the ankle-joint fully extended, and the toes turned in. When the patient, in the act of walking, placed the heel on the ground and allowed the weight of the body to fall on the advanced leg, the sole of the foot came down all at once with a flop?-flop, and pain was felt at each step taken with this foot. The pain was referred to a spot corresponding to the origin of the peroneus tertius muscle. The patient found it impossible to stand 011 the heel of the left foot: every attempt to do so resulted in the sole of the foot coming down flop on the ground, and was accompanied by intense pain in the part of the leg I have referred to. O11

carefully testing the condition of the extensor longus digitorum, extensor proprius hallucis, and tibialis anticus muscles, I could discover no abnormality; the power of these muscles to resist flexion of the toes and extension of the ankle was as great as usual, and 110 pain was elicited by such testing. This condition of

paralysis continued for about three days, when the muscle gradu- ally regained its power. During that time walking was practised by keeping the knee slightly more bent than usual, turning the toes slightly more outwards, and placing the sole of the foot 011

the ground at each step of the left foot. In this way there was no

pain accompanying the act of walking, and the difference of gait was scarcely perceptible. The second case was one in which the patient, on descending from

a high leap, alighted on the ground 011 his right heel, the ankle

1892 ] FUNCTION OF THE FEItONEUS TERTIUS MUSCLE. 633

being at the time extended. The symptoms in this case were similar to those in the former, only they were more aggravated; and examination showed that the extensor longus digitorum, particularly the outer part of it, was also involved in the strain. The power of this extensor to resist flexion of the toes was impaired, and the pain in the leg was correspondingly increased in its distribution. The act of walking was performed as in the former case.

These two cases illustrate very clearly the action of the peroneus tertius as a muscle of ordinary walking when the heel is the first part of the foot to touch the ground. Acting with the tibialis anticus, and aided by the extensors of the great and other toes, it prevents the foot from coming forcibly down on the ground, which it tends to do, as soon as the weight of the body bears 011 the ankle- joint ; in other words, these muscles on the front of the leg prevent "spasmodic" extension of the ankle-joint in ordinary walking. The action of this muscle, too, in helping to maintain the erect posture of the body becomes apparent; and this action goes very far to explain, if it does not make perfectly clear, how the peroneus tertius muscle is the peculiar property of the human subject, no other mammal possessing it.

I have not yet seen an instance of the absence of this muscle, and I am unable to say what the state of matters may be in such

instances; but it would be instructive to know the condition of the slip of the extensor longus digitorum to the little toe, and whether any peculiarity exhibited itself in the gait of persons in whom the peroneus tertius was absent.