A FROG 189 



narrow tendinous band called the linea alba lies in the median 

 line and separates the right from the left rectus. There are also 

 present in these muscles four or five transverse, tendinous bands 

 which divide them into segments. This segmentation, which 

 also appears in the rectus abdominis of many higher vertebrates, 

 including man, is an inheritance from the metameric condition 

 of the body muscles in the fishes and the salamanders. 



The external oblique muscle forms the lateral wall of the abdo- 

 men on each side. It is a broad, thin muscle which extends from 

 the mid-dorsal to the midventral aponeurosis, its fibers having 

 an oblique direction. Immediately beneath this muscle is the 

 transversus, the fibers of which have a transverse direction. 



Exercise 21. Draw the ventral aspect of the body showing these 

 muscles. 



Study the superficial muscles of the ventral surface of the hind 

 leg. The longest muscle of the thigh is the sartorius. It is a long 

 band which extends along the middle of the thigh from the pel- 

 vis to the proximal end of the shank. Just in front of it is a 

 broad muscle, the vastus internus, which forms the anterior bor- 

 der of the thigh. It forms also the anterior portion of a three- 

 fold muscle, the triceps extensor femoris, which is the principal 

 extensor muscle of the thigh. The other two portions of this 

 muscle are on the upper side of the leg ; they are the rectus an- 

 ticus femoris and the vastus externus, the latter being posterior 

 to the former. 



Posterior to the sartorius on the ventral surface are three mus- 

 cles, the adductor magnus, the rectus internus major, and the 

 rectus internus minor, the latter of which forms the hinder 

 margin of the thigh. These are all, together with the sartorius, 

 flexors of the leg. 



On the lower leg, or shank, the large muscle which forms the calf 

 is the gastrocnemius. At its lower end is the tendon of Achilles, 

 which passes over the ankle and is continued in the plantar apo- 

 neurosis, a broad, tendinous band covering the sole of the foot. 

 The front side of the shank is formed by the tibialis anticus 

 muscle. 



