1903.} THE MUSCLES OF THE UXGULATA. 271 



Elephant (81, 85), however, an expansion from this insertion is 

 found running down to the inner side of the ankle. According 

 to some authors the semimembranosus may occasionally be absent 

 altogether : this was the case in Kinberg's Chevrotain (20), 

 Ohatin's Water- Chevrotain (24)*, and Lesbre's Horse (65). The 

 presemimembranosus is usually larger than the semimembranosus, 

 and is inserted by fleshy fibres into the lower part of the femur 

 just above the internal condyle; besides its fusion with the semi- 

 membranosus it is very intimately connected with the adductor 

 mass and so has escaped notice by some observers. The nerve- 

 supply in the Pig, Antelope, Sheep, and Hyrax is from the nerve 

 to the hamstrings, a large branch which comes ofl" the internal 

 popliteal element of the great sciatic. 



Semitendinosus. — This muscle, like the last, may rise by one or 

 two heads, of which the one from the tuber ischii is always present. 

 A caudal head is rarely found in the Artiodactyla, Chatin's Water- 

 Chevrotain (24) and the Pig (4, 6) are the only exceptions we 

 know. In the other subordei-s, the Horse (63, 64, 65), Tapir (58, 

 62), and Hyrax (74, 75, 78) have two heads (see text-figs. 24 <fe 

 26, S.T.). The insertion is into the second quarter of the 

 internal surface of the shaft of the tibia as a rule, and from 

 this a small expansion is sent ofi" to help the biceps and gracilis 

 in reinforcing and providing a sheath for the tendo Achillis, 

 and so acting on the calcaneal tuberosity. These expansions 

 from the inner and outer hamstrings are seen at their best 

 in the TJngulata ; but they are so common among the Mammalia 

 generally t, that they probably have a phylogenetic history to 

 account for their presence, as well as a physiological advantage 

 to account for their special development in Ungulates. In all 

 probability their past history is that they are remnants of the 

 longitudinal muscle-fibres passing uninterruptedly from the 

 pelvic girdle to the foot, such as are found among the Reptilia, 

 while the explanation of the fact that they are so well developed 

 in Ungulates miist be looked for in the mode of life of these 

 animals. Among all the orders of mammals there is not one 

 which is more distinctly cursorial in the habits of its species; 

 with the exception of the Tree-Hyrax there are no arboreal types ; • 

 the Hippopotamus and Water-Chevrotain are the only ones which 

 are partially aquatic, while there is no example of a fossorial 

 Ungulate. For these reasons, any modification which we meet 

 with in the limb-muscles of Ungulates as a group, will probably be 

 explained by its being an advantage to them in running. The 

 expansions of the hamstrings to the calcaneum act as plantar 

 flexors of the ankle-joint, while the hamstrings themselves are 

 flexors of the knee. In watching the sequence of movements of 



* In our specimens of Chevrotain (21, 22) tlie muscle was undoubtedly present, 

 though so closely connected with the presemimembranosus that it might easily be 

 overlooked without careful dissection of the insertion. 



t The fascial expansions from the semitendinosus and gracilis in Man will at once 

 occur to the human anatomist. 



