I go PHYLUM CHORD ATA 



Exercise 22, 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 23. 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 ver- 

 tebrates consists of the hardened bony or horny structures which 

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

 imal. These structures are very poorly represented in the frog. 

 The skin is naked, no bony or horny scales or other hardened in- 

 tegumental 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. 



The endoskeleton consists of the bony and cartilaginous frame- 

 work of the body. It may be divided into the axial skeleton, 

 which includes the skull and the spinal column, and the appen- 

 dicular skeleton, which includes the framework of the two pairs 

 of appendages, that is, the legs and the girdles which join them 

 with the trunk. The breastbone may also be conveniently studied 

 with the appendicular skeleton. 



The Appendicular Skeleton. The anterior appendages consist 

 of the forelegs and the pectoral girdle. This girdle is formed of 

 a right and a left half which meet midventrally ; here they enter 



