THE MUSCULAR SYSTEM 229 



posterior limbs, inserting the one into the ischium, another 

 into the femur, and the third, into the margin of the pubo- 

 ischio-tibialis. 



Unlike the anterior limb, in which the most of the muscles 

 occurring in Necturus may be readily recognized in mammals, 

 the homologies of the proximal portion of the pelvic limb are 

 all more or less doubtful. As a beginning, there are found 

 upon the outer side of the pubo-ischium in mammals the 

 obturator externus, the adductor es, the gracilis, which -belongs 

 with the adductor group, and possibly the sartorius. Of these 

 the obturator is probably the homologue of the pubo-ischio- 

 femoralis externus, and the remainder may be derivatives of 

 the pubo-ischio-tibialis, in spite of the difference in respect to 

 insertion. The pubo-ischio-femoralis internus seems to give 

 rise to the obturator internus -with the two associated gemelli, 

 as well as to the ilio-psoas complex, which appears first as a 

 distinct muscle in reptiles. The ilio-extensorius is probably the 

 prototype of the great complex of the front of the thigh, quad- 

 riceps femoris f composed of the three vasti, externus, medialis 

 \_crur eus~\ and internus, and the rectus femoris. The glutai 

 are probably derived from the ilio-femoralis. 



Of the muscles of the posterior aspect of the thigh, enor- 

 mously developed in mammals, the inner ones, Mm. semimem- 

 branosus and semitendinosus, are probably also derived from 

 the pubo-ischio-tibialis, while the two heads of the outer one, 

 the biceps femoris, come from two distinct sources, and in 

 many mammals are separate muscles. The derivation of the 

 long head is uncertain, but it may be homologous with the ilio- 

 fibularis of urodeles, in spite of the difference in origin. The 

 short head, on the other hand, is derived from the glutaeal 

 group, and is identical with the long narrow band, described 

 in many mammals as the teniiissimus. Since it is associated 

 with the long head to form a " biceps " muscle in a few mam- 

 mals only, including man and several apes, this slip is best 

 considered as a separate muscle of the glutaeal group, under 

 the name of glutceo-cruralis. The caudal group of muscles, 



