THE FROG 119 



Back of the breastbone are the abdominal muscles, which form 

 the ventral and lateral walls of the abdomen. Three pairs are 

 present,— the rectus abdominis, the external oblique, and the 

 transversus. 



Extending from the hinder end of the breastbone to the hinder 

 end of the trunk is the aponeurosis just mentioned, a broad 

 median band of connective tissue covering the midventral area 

 of the abdomen. A similar aponeurosis is also present in the 

 mid-dorsal area. 



The rectus abdominis muscles are a pair of longitudinal muscles 

 which lie in the midventral area beneath the aponeurosis. A nar- 

 row 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 appears also 

 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 caudate amphibians. 



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 trans- 

 versus, the fibers of which have a transverse direction. 



Exercise 25. 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 pelvis to the proximal end of the shank. Just in front of it 

 is a broad muscle, the vastus internus, which forms the ante- 

 rior border of the thigh. It forms also the anterior portion of a 

 threefold muscle, the triceps extensor femoris, which is the prin- 

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

 muscle are on the upper side of the leg ; they are the rectus anticus 

 femoris and the vastus externus, the latter being posterior to 

 the former. 



