Bischoff at first denied the existence of the plantaris in the chimpanzee, and Brühl had previously done the same, but it is as normally present in that animal as in man, in whom also it is sometimes absent. I, however, as well as other observers, have failed to discover it in the gorilla, orang, and gibbon. The popliteus is developed in every instance. The tibio-fibular muscle (_Musculus peroneotibialis_), covered by the popliteus, of which the existence was ascertained by Gruber, has not been observed by me in any of the anthropoids, with the exception of the chimpanzee. But it was very apparent in a red sea-cat monkey (_Cercopithecus ruber_). The gastrocnemius, which is easily separable into two heads, and the peroneal muscles have not the same relative width in anthropoids and man, since in the former case the calf of the lower limb is small, and it lacks the pleasing roundness which characterizes this part of the human structure. These muscles, especially in the orang and gibbon, appear to take a lateral direction. The Tendo Achillis is present, but it has not the prominent development in height and width which we observe in man. The long extensor, flexor, and tibial muscles are in all cases fully developed. The peroneus tertius, as it is termed, although it should only be regarded as a part of the extensor longus digitorum, is absent in anthropoids.[74] I myself am not disposed, with Huxley, Bischoff, and others, to regard this muscle as an abductor. Brühl perceived in a chimpanzee a fourth rudimentary peroneal muscle (_Musculus peroneus intermedius_), extending between the peroneus and the little toe, a muscle sometimes found in man, and which I have myself only observed in one adult chimpanzee. In the gorilla and the chimpanzee the extensor longus digitorum passes through a remarkably strong transverse ligament, formed of fibrous cartilage, which covers the tarsus. It acts upon the four outer toes (Fig. 55). Brühl has described the characteristic contraction and extension of the tendons of the long and short extensors of the toes in the chimpanzee, but I have myself found some difficulty in producing this action. In Fig. 55 I have endeavoured to represent this condition in the most natural way. The extensor proprius pollicis is in all cases developed. The extensor brevis digitorum produces a large, oblique belly for the great toe (Fig. 55). In the gorilla there is for the great toe an abductor, a bicipital flexor, an adductor, and an opponens (comp. Fig. 54). From the extensor brevis digitorum the belly for the great toe rises with a certain independence. On the right foot of a chimpanzee I observed a fifth belly of this muscle, going to the little toe (Fig. 55). As my illustration is taken from this specimen, I have represented the foot with, or in spite of, this interesting anomaly, which, as we know, sometimes occurs in man. [Illustration: