120 VERTEBRATE ZOOLOGY 



Posterior to the sartorius on the ventral surface are three muscles, 



the adductor magnus, the rectus interims 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 aponeu- 

 rosis, a broad tendinous band covering the sole of the foot. The 

 front side of the shank is formed by the tibialis anticus muscle. 



Exercise 26. Draw the ventral aspect of the hind leg. 



Study the superficial muscles of the dorsal surface of the leg. 

 The anterior half of the surface of the thigh is occupied by two 

 muscles already noted, the rectus anticus femoris and the vastus 

 externus. Posterior to the last-named muscle are the biceps 

 femoris, the semimembranosus, and the rectus internus minor, 

 the latter forming the hinder margin of the leg. 



On the shank will be seen the large gastrocnemius, forming the 

 calf of the leg ; the tibialis anticus, forming its anterior border ; 

 and the peroneus, between the two. 



Exercise 27. Draw the dorsal aspect of the leg. 



Without injuring the bones trace each of these muscles to its 

 origin and insertion and determine which muscles are extensors 

 and which are flexors. 



The Skeletal System. This system is made up of two portions, 

 the exoskeleton and the endoskeleton. The exoskeleton in verte- 

 brates consists of the hardened bony or horny structures which 

 develop in the skin and furnish an external protection to the ani- 

 mal. These structures are very poorly represented in the anuran 

 amphibians. The skin is naked, no bony or horny scales or other 

 hardened integumental structures being present. The toes are also 

 without claws. The only exoskeletal structures are the teeth and 

 certain bones, called membrane bones, which form a part of the 

 skull. These bones are, however, so intimately joined with the 

 other bones and cartilages of the skull that they will be studied 

 in connection with them. 



