THE PERONEUS TERTIUS MUSCLE I N GORTLLAS DUDLEY J. MORTON Milford, Connecticut T W O FIGURES The peroneus tertius muscle has always enjoyed the distinction of being an exclusively human structure, and as such it has helped to separate man from the lower animals. I n two dissections of gorilla feet, however, which the writer has had the opportunity to make, this muscle was definitely present. The first gorilla, a young male about five and one-half years old, had been brought to this country early in 1931 by the Ringling Brothers’ Circus. It died shortly after its arrival, and the body was given to the American Museum of Natural History, New Pork City. The second specimen was a pair of unskeletonized feet of an adult female gorilla, which had been brought with other gorilla material from the Belgian Congo by Mr. Carl E. Akeley, of the American Museum of Natural History, the following year. Sir Arthur Keith, in his article on “The Adaptional Machinery concerned in the Evolution of Man’s Body,” makes the following statement: “ I n the gorilla one notices occasionally a tendency for the outer fibres of the tendon (extensor longus digitorum) going t o the fifth toe to stray or migrate toward the outer border of the foot.” It may seem strange, therefore, in view of the extensive studies of the anthropoid apes made by this noted English anatomist, that the two specimens under consideration should 323 324 DUDLEY J. MORTON show such a marked resemblance to the typical human structure. The accompanying illustrations, however, show a far more advanced condition than the one described by Sir Arthur Keith. Prof. George S, Huntington, of the New York College of Physicians and Surgeons, found in the feet of the younger animal the condition pictured in figure 1. The writer was extended the privilege by Professor Huntington of working with him in the dissection. In this specimen there were two distinct tendinous cords extending from the common belly of the extensor longus digitorum muscle to the fifth digit. They were easily separable well up into the muscle belly. The anterior (extensor) tendon was inserted into the distal phalanx after the manner of the other long extensor tendons; it also gave off a light fascial band which was attached to the fifth metatarsal bone in the anterior portion of its base. The posterior (peroneal) tendon had its primary insertion by a similar but somewhat heavier fascial band to the fifth metatarsal base; it also continued forward as a thin white cord under the fibrous band of the extensor tendon, to blend with the deep fascia on the outer side of the digit, similar to the insertions of the short extensor tendons on the other toes. The two tendons and fascial bands were quite independent, except near the lower end of the muscle belly. The pair of adult gorilla feet, which had been disarticulated at the level of the ankle-joint, were kindly assigned to the writer’s use by Mr. Akeley and Prof. W. K. Gregory, of the Museum, for his investigations into the evolutionary development of the human foot. I n the subsequent dissection, a pair of peroneus tertius tendons were disclosed, which seem to exclude any possible classification of the previous specimen as a chance variant or anomaly (fig. 2). I n the adult feet there was no paralleling of the outer extensor and peroneal tendons, and no fascial band united the extensor tendon with the metatarsal bone. The peroneal tendon (marked with the arrow) sprang from the common extensor PERONEUS TERTIUS MUSCLE I N GORILLAS 325 Fig. 1 Right foot of 5-oung male gorilla, showing what is apparently a formative stage in the eTolutionary development of the peroneus tertius muscle (indicated b y arrow). 326 DUDLEY J. MOIlTON muscle belly. It was smaller than the extensor tendons, closely comparable in relative size to its usual proportions in man, and elliptical in its cross-section. The insertion mas located in the outer, upper aspect of the base of the fifth metatarsal bone. Just above this insertion, as faintly seen in the photograph, a thin, thread-like prolongation is carried forward to the digit, at a right angle to the tendon proper. Fig. 2 Left foot of adult female gorilla with peroneus tertins tendon, presenting a typically human stage of development. It is interesting to note, from Piersol’s textbook on “Hnman Anatomy, ” Prof. J. Playfair MchIurrich ’s comments on the variation of this muscle in man. “The peroneus tertius is quitely frequently absent, and is usually more or less closely united with the extensor longns digitorum above. Its tendon sometimes splits into two portions, the additional one passing either to the fifth toe o r to the fourth metatarsal. “Notwithstanding its name, which has reference to its origin from the fibula, the peroneus tertius has morphologically nothing to do with the other peroneal muscle, but is a PERONEUS TEBTIUS MUSCLE I N GORILLAS 327 separate portion of the extensor longns digitorum, whose connections with the metatarsals are interesting in this regard. ” The recognizable inconstancy cf the peroneus tertius in man, and its presence in the gorilla alone of the anthropoid apes as well as of all other primates and mammals, points to two interpretations : 1)that the muscle is a comparatively recent acquisition of the human stock, not antedating man’s adoption of ground life, and, 2) that it is the product of bipedal terrestrial usage. The gorilla is not an arboreal animal as were its ancestors, but an habitual ground dweller, and although its posture is characteristically semierect and its gait not completely bipedal, nevertheless the feet are distinctly plantigrade and bear a greater share of body weight than do the hind feet of digitigrade quadrupeds. Terrestrial modifications of the gorilla’s foot skeleton are seen to have attained an advanced degree of development when compared with the foot of the arboreal chimpanzee. The differences in bone structure have been fully considered in the reports on the research referred to above. I n connection with those studies, it seems very obvious that the appearance of the peronens tertius muscle in the gorilla constitutes the natural response of the soft structures to a profound change in function induced by an altered environment---a response that was correlated with the changes in bone and which parallels the process which occurred at an earlier period in the foot of man. When we consider the range between Sir Arthur Keith’s statement and the human-like condition found in Mr. Akeley ’s adult gorilla, the intermediate stage manifested by the younger gorilla would seem to attach particular value to these findings as they pertain to muscle migration. The basal metatarsal insertions were apparently not acquired by a ‘creeping’ migration of the original insertions, but by a direct projection of a new group of tendinous fibers along a fascia1 plane t o the point of maximum mechanical advantage for the function required-elevation of the outer border of the foot. 328 DUDLEY J. MORTON LITERATURE CITED KEITH, SIRARTHUR 1923 Nature (England), no. 2807, Aug. 18. MORTON,DUDLEYJ. 1922 Evolution of the human foot. Am. J. Phys. Anthrop., vol. 5, no. 4, Oct. 1924 Am. J. Phgs. Anthrop., vol. 7, no. 1, J a n . 1924 Evolution of the longitudinal arch of the human foot. Jour. Bone and Joint Surg., vol. 7, no. 1, Jan. 1924 Mechanism of the normal foot and of flat foot. Jour. Bone and Joint Surg., vol. 7, no. 2, Apr.