THE FROG 



195 



from the brain (efferent fibers), and the posterior root carries 

 impulses toward the brain (afferent fibers). 



The Sympathetic System. In the abdominal cavity, lying 

 on each side of the spinal column, is a chain of minute nerve 

 ganglia, ten in number, which are also 

 connected with the spinal nerves; Fig. 

 95 sy. These constitute the sympa- 

 thetic system. From these two chains 

 of ganglia minute nerves are given off, 

 chiefly to supply the intestine, the kid- 

 ney, and the other organs of the ab- 

 domen. Although connected with the 

 spinal nerve, the sympathetic system is 

 quite distinct and has special functions. 



The microscope shows that the nerv- 

 ous system, like that of the earthworm, 

 is composed of an enormous number of 

 neurons, each with its cell body, dendrites, 

 and axon. These are massed in the 

 brain and cord, and there are many also 

 in the ganglia outside of the cord. They 

 are so situated that part of them carry 

 impulses to the center, and part of them 

 carry them in the reverse direction. 

 Their numbers are greater and their re- 

 lations more complex than those of the 

 earthworm. 



The Sense Organs. At the periph- 

 eral end of all of the sensory nerves 

 are found very complicated organs, con- 

 structed so as to be affected by certain 

 external stimuli. When they are stimu- - (Modified from Parker.) 

 lated impulses start from them and pass 



over the afferent nerves to the brain, where they become 

 sensations. They are the sensory organs and are as follows: 



VII 



VIL 



FIG. 95. DIAGRAM 



SHOWING THE RELA- 

 TION OF SYMPATHETIC 

 SYSTEM TO THE 

 SPINAL NERVES 



The sympathetic chain of 

 one side only is shown. The 

 spinal nerves are indicated by 

 Roman numerals; sy, sympa- 

 thetic nerve; syg, sympathetic 

 ganglia. 



