loo FIRST LESSONS IN ZOOLOGY 



sensitive to light; in the internal ear there are certain 

 cells which are affected by sound vibrations; in the nasal 

 passages there is a region in which are cells sensitive 

 to odors ; in the skin of the tongue are cells that react 

 to sweet, sour, and bitter liquids; in various parts of the 

 skin are cells sensitive to pressure, heat, and cold. These 

 different kinds of cells affected by different influences are 

 called sense-cells. 



Now what the animal sees, hears, touches, etc., deter- 

 mines its motions, and we find that the sense-cells are 



Fig. 65. — Central iK-rvcus system of a dog. (After Ritzema-Bos.) 



connected with the muscles by means of the nervous 

 system. Through this connection light, heat, sound, etc., 

 guide muscular action. For example, the hawk's eye is 

 connected through the nervous system with the wing and 

 leg muscles of the bird, and by this means the wings and 

 legs may be made to make the motions necessary to catch 

 the chicken. To understand the makeup of the nervous 

 system, that of some small vertebrate should be examined 

 in connection with the following description. 



The central nervous system. — The nervous system of a 

 vertebrate (fig. 65), consists of a central portion, the brain 

 (fig. 66), and spinal cord, from which branches called 

 nerves extend in pairs ; the nerves then branch and branch 

 again until their divisions reach every part of the body in 



