350 BODY CONTROL AND HABIT FORMATION 



will, is known as the nervous system. In the vertebrate animals, 

 including man, it consists of two divisions. One includes the 

 brain, spinal cord, the cranial and spinal nerves, which together 

 make up the cerebro-spinal nervous system. The other division is 

 called the sympathetic nervous system and has to do with those 

 bodily functions which are beyond our control. Every group of 

 cells in the body that has work to do (excepting the floating cells 

 of the blood) is directly influenced by these nerves. Our bodily 

 comfort is dependent upon their directive work. The organs 

 which put us in touch with our surroundings are naturally at the 

 surface of the body. Small collections of nerve cells, called ganglia, 

 are found in all parts of the body. These nerve centers are con- 

 nected, to a greater or less degree, with the surface of the body by 

 the nerves, which serve as pathways between the end organs of 

 touch, sight, taste, etc., and the centers in the brain or spinal cord. 

 Thus sensation is obtained. 



Sensations and Reactions. - - We have already seen that simpler 

 forms of life perform certain acts because certain outside forces act- 

 ing upon them cause them to react to the stimulus from without. 

 The one-celled animal responds to the presence of food, to heat, to 

 oxygen, to other conditions in its surroundings. An earthworm is 

 repelled by light, is attracted by food. All animals, including man, 

 are put in touch with their surroundings by what we call the or- 

 gans of sensation. The senses of man, besides those we commonly 

 know as those of sight, hearing, taste, smell, and touch, are those of 

 temperature, pressure, and pain. It is obvious that such organs, 

 if they are to be of use to an animal, must be at the outside of the 

 body. Thus we find eyes and ears in the head, and taste cells 

 in the mouth, while other cells in the nose perceive odors, and 

 still others in the skin are sensitive to heat or cold, pressure or 

 pain. 



But this is not all. Strangely enough, we do not see with our 

 eyes or taste with our taste cells. These organs receive the sensa- 

 tions, and by means of a complicated system of greatly elongated 

 cell structures, the message is sent inward, relayed by other elon- 

 gated cells until the sensory message reaches an inner station, in 

 the central nervous system. We see and hear and smell in our 



