324 AMPHIBIA xn. 12- 



phalanges. Flexor digitorum brevis muscles arise from this tendon 

 for insertion on the digits. 



Of the dorsal muscle mass (Fig. 187) the latissimus dorsi and 

 dorsalis scapulae are the most proximal, running from the middle of 

 the back to the humerus and serving to abduct and draw back the 

 whole limb. The triceps (anconeus) serves to extend the elbow; in 

 the forearm are extensor carpi ulnaris and radialis and extensors for 

 the fingers. 



According to this plan protractor (flexor) muscles lie mainly an- 

 terior to retractors (extensors), corresponding to the ancient move- 

 ment by which the limb was drawn first forward then back as a swim- 

 ming wave passed down the body. In all tetrapods flexor muscles are 

 in general innervated by spinal roots anterior to those for the exten- 

 sors. The locomotory movements of the limbs therefore still show the 

 passage of an excitation wave backwards along the spinal cord, a relic 

 of the swimming rhythm of fishes. However, the changes that have 

 taken place in the relative positions of the parts of the limbs make it 

 difficult to follow out this simple pattern in detail. It must also con- 

 tinually be remembered that many muscles produce rotation as well 

 as movement in the main planes of the body. 



In the hind limb, muscles of the same two general types can be 

 recognized, namely anterior muscles, which draw the limb forward 

 and flex and adduct its joints, and posterior ones, which draw it back 

 and extend and abduct. The specialization of the main muscle masses 

 has gone much farther, however, so that more individual muscles are 

 found, especially round the hip joint, each serving to move the limb 

 in a special way. 



In the thigh (Figs. 187 and 190) the muscles of the anterior group, 

 lying on the ventral surface, are the pectineus and the adductors, 

 running from the pelvic girdle to the femur and thus serving to move 

 the whole limb inwards (adduction). The sartorius, biceps, semi- 

 membranosus, and semitendinosus are two-joint muscles mainly pro- 

 ducing flexion at the knee as well as at the hip. 



The more posterior and dorsal group of muscles includes the 

 gluteus and tensor fascia lata from girdle to femur, extending the 

 thigh joint, and the very large cruralis (including the rectus femoris 

 and triceps femoris) running from girdle and femur to tibia. This is 

 the main extensor of the knee, being helped by gracilis and semi- 

 membranosus. This extension is obviously an important part of the 

 jumping movement of the frog. 



In the shank the arrangement of the flexors and extensors into the 



