94 



Basic Structure of Vertebrates 



being opposed by a ventral depressor (or flexor). In some cases 

 there is an anterior abductor which causes the bladelike pectoral fin 

 to project from the body so that the anterior edge of the fin "cuts" the 

 water; and there may be a posterior adductor which draws the fin 

 back flat against the side of the body. 



The leg of a tetrapod, in contrast to the one-segment fin, con- 

 sists of three major segments — the upper arm, forearm, and hand 



Scapula 



Fig. 94. Relations of the biceps and 

 triceps of the mammalian arm. Contrac- 

 tion of the biceps raises or flexes the fore- 

 arm at the elbow joint. Contraction of 

 the triceps lowers or extends the forearm. 

 The distal end of the humerus serves as a 

 fulcrum on which the bones of the fore- 

 arm (radius and ulna ) work as a lever. 



of man; the thigh, shank, and foot. The foot and hand contain nu- 

 merous minor segments. The leg as a whole, like the fin of a fish, may be 

 moved in relation to its girdle. But the numerous joints within the leg 

 itself make possible a great variety of movements of one part of the 

 appendage in relation to other parts. The effecting of these move- 

 ments requires many muscles, and their arrangement is necessarily 

 complex. In contrast to the three or four muscles of the fin of a fish, 

 there are more nearly three or four scores of them in one appendage of 

 a mammal (Fig. 91). 



