66 THE ANIMALS AND MAN 



which branches called nerves extend in pairs; the nerves then 

 branch and branch again until their divisions reach every part 

 of the body in the shape of very numerous white threads, tco 

 small to be detected by the naked eye. These very small 

 nerve-threads or fibers end at last in connection with cer- 

 tain of the tissue-cells. All the sense-cells of the retina, 



Fig. 24. Central nervous system of a dog. (After Ritzema-Bos.) 



ear, nose, tongue, and skin are connected with minute 

 nerve-fibers as are also all the muscle-fibers. Now all the 

 nerve-fibers from both sense-cells and muscle-cells run to 

 the central portions of the nervous system, the brain and 

 spinal cord, and are there in some way definitely connected 

 with one another, thus making pathways over which every- 

 thing that affects the eye, ear, and other sense-organs may 

 affect the muscles. 



The nervous system of all vertebrates is on the same 

 general plan, being, however, less complex in the lower 

 forms. All animals with a definite nervous system have 

 nerve-fibers connecting both sense-cells and muscle-cells 

 with certain central parts. They differ, however, in the 

 arrangement of these parts. And since they differ also in 

 muscular arrangement, and in the kind and position of the 

 sense-organs, the arrangement of the nerve-fibers connecting 



