190 STRUCTURE OF THE VERTEBRATES 



the first urodeles the pelvic limbs were used for propulsion. When 

 the knee was drawn toward the anterior part of the body, the 

 torsion placed the dorsal muscles of the thigh in an anterior 

 position. With this change the origins of the pelvic extrinsic 

 muscles became compressed into narrower limits. This is par- 

 ticularly true of man and the higher anthropoids, where a more 

 or less upright walking position has been assumed. In the mam- 

 mals the rectus femoris on the anterior of the thigh; the ad- 

 ductor magnus which lies laterally; and the large gluteus group 

 which form the buttocks, can be compared in position with the 

 three dorsal muscles of the pectoral limb. In the lumbar region 

 a large muscle mass migrates into a sub-skeletal position, with 

 origins on the ventral side of the transverse processes of the 

 vertebrae. This is a very constant group in all land vertebrates, 

 and is the ilio-psoas group of the mammals. They insert on the 

 femur near its head. 



The intrinsic pelvic muscles are also more modified than the 

 pectoral, particularly the more distal ones. In most vertebrates 

 the foot is held in a permanent position of dorso-flexion. There- 

 fore the dorsal (extensor) group increase the flexion of the foot 

 against the shank; and conversely, the ventral (morphologically 

 the flexor) gastrocnemius and other calf muscles pull the foot 

 into a straight line with the limb. Reference should be made to 

 the discussion of the evolution of the leg position. The calf 

 muscles are usually much more powerful than the dorsal group. 

 In the normal walking position of the vertebrate the push of the 

 pelvic limb is gained by straightening the three angles — that be- 

 tween body and thigh, thigh and shank, and the shank and foot. 

 The last gives the final push forward, particularly when the 

 animal is in rapid motion. 



F. Integumentary Muscles 



The muscle slips which become attached to the skin arise 

 from all three of the skeletal muscle groups. The integumentary 

 muscles are of small importance when they occur in the reptiles, 

 but they become quite prominent in most of the orders of mam- 

 mals. A dog or horse can shake his skin without movement of 

 the skeletal muscles. In the armadillo, which is covered with 



