THE ANIMAL ORGANISM 



37 



The skeleton of the frog consists of bones which serve as a 

 supporting framework for the body. Bodily movement is 

 brought about by the contractions of muscles which are located 

 external to the skeleton. The body 

 musculature is composed of numerous 

 distinct bundles of muscle tissue each of 

 which is known as a muscle, whose action 

 is under the control of the central nervous 

 system. Joints between skeletal ele- 

 ments make possible the movement of 

 different portions of the body. The 

 extension or flexion of a segment of a 

 limb, for example, is brought about 

 through the action of two antagonistic 

 sets of muscles extending over the joint, 

 the contraction of one set producing an 

 effect opposite to that of the other. The 

 muscles are attached to the bones by 

 means of tendons and it is usually the 

 tendon from the distal end of the muscle 

 that extends over the joint to be inserted 

 on the bone. A muscle produces move- 

 ment by contraction, which draws its 

 ends, and any parts attached to them, 

 toward each other. Relaxation of the 

 muscle is passive and does not produce a 

 pushing effect. 



The frog has a central nervous system fig. 16— Dorsal view of 

 consisting of the brain, enclosed in the c D entral nervous s y s 4 tem ° f 



a • 7 j R ana catesbeiana. A, audi- 



Cranium Of the skull, and a spinal COrd tory nerve; C, cerebellum; 



lying in the channel formed by the neural g H - ce £,ebrai hemisphere; 



J & J E, eye; F, filum terminate; 



arches of the vertebrae of the backbone, i.e., internal ear; M, me- 

 Connected with the brain are ten pairs d , u f lla ; N * nasal cavity: .?• 



. olfactory nerve; Op, optic 



of cranial nerves distributed mainly to nerve; O.L., optic lobe; P, 

 various parts of the head. The tenth P in f \ organ; S.C spinal 



^ m cord; 1,2,7,8,9, spinal nerves. 



cranial nerve, the vagus, has unportant 



connections with the viscera. There are also ten pairs of spinal 

 nerves connected with the spinal cord which are distributed to the 

 trunk and limbs. The chain ganglia of the sympathetic nervous 

 system consist of a pair of nerve trunks which extend from within 



-7,8,9 



