256 PRACTICAL ZOOLOGY 



together. Each leg contains a thigh bone or femur, a leg bone or 

 tibiofibula, four small ankle bones, five rows of bones in the 

 digits, and an extra digit bone, the prehallux. 



Various kinds of joints are represented in skeleton of the ver- 

 tebrates. Some of these are immovable, such as those of the cra- 

 nium ; others are movable. The fore limbs and hind limbs form 

 ball-and-socket joints with the pectoral and pelvic girdles; the 

 knee and elbow joints work like a hinge; the bones of the wrists 

 and ankles form gliding joints; and the bones of the forearm 

 (radius) in some vertebrates form a sort of pivot at the elbows. 



Muscular Activity. As already pointed out the muscles 

 consist of specialized contractile cells and are the agents of 

 active movement. The " flesh " of vertebrates is largely mus- 

 cle. As a rule these muscles are attached by one or both ends 

 to bones either directly or by means of bands of connective 

 tissue, the tendons. Movements depend upon the attachment 

 of the muscles and the kinds of joints between the bones. 



Most of the large muscles of the frog are used in leaping and 

 are consequently in the hind limbs (Fig. 146). 



A few of these are as follows : (i) The sartorius bends the 

 hind leg, drawing it forward and ventrally; (2) the gastrocne- 

 mius bends the hind leg and extends the foot; (3) the adductor 

 magnus bends the thigh ventrally ; (4) the rectus internus major 

 bends the hind leg; and (5) the peroneus extends the hind leg 

 and foot. The pectoralis major moves the fore limbs. 



These are all voluntary muscles. Most of the muscles of the 

 interior organs are involuntary; those in the wall of the bladder 

 are excellent examples of this type and can be examined easily. 



Nervous Activity. The nervous system of vertebrates is 

 more complex than that of any other animals. In fact, man 

 owes his dominance over other animals to the great develop- 

 ment of his brain. The central nervous system consists of the 

 brain and spinal cord; the peripheral nervous system consists 

 of the cerebral and spinal nerves; and a sympathetic system is 

 also present. 



