ANIMAL PHYSIOLOGY 



65 



yet we may at least examine some of the parts of the body 

 specially concerned with these processes. In the higher 

 animals they are determined and directed by means of the 

 sense-organs and the nervous system. In vertebrates the 

 special senses, as they are called, are those of sight, hearing, 

 smell, taste, touch, cold, heat, and one called the muscular 



Sunfish. Toad. Snake. Sparrow. Mouse. 



olf. I. 

 cbr. 



md. br. 



Fig. 23. Diagram of brains of vertebrates; olf. I., olfactory lobes; cbr., 

 cerebrum; mi. b .,%nidbrain (optic lobes); cbl., cerebellum; med. 

 ob., medulla oblongata; sp. cd., spinal cord. 



sense. A part of the eye known as the retina is specially 

 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, soirr, 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 con- 

 nected with the muscles by means of the nervous system. 

 Through this connection light, heat, soimd, etc., guide 

 muscular action. 



The nervous system of a vertebrate (fig. 24), consists of a 

 central portion, the brain (figs. 23, 25), and spinal cord, from 



