212 BIOLOGY 



ing at their ends the individual telephones which may receive com- 

 munications from the central system or send messages to it. 

 So the central nervous system contains the intelligent, originat- 

 ing force, and being in communication with every part of the 

 body, controls all of the functions in such a way that they act in 

 harmony. This central system has a series of efferent nerves, 

 by which it sends messages outward, and a series of afferent 

 nerves, by which messages are brought inward to the brain. 

 The most important of the latter are the sensory nerves. 



Sense organs. Each sensory nerve ends in a sense organ, so 

 formed that it is excited by definite external stimuli One of 

 them, the ear, is stimulated by vibrations of the air; another, 

 the eye, by vibrations of ether; others by a slight pressure or 

 touch; others by heat; others again, by chemical substances, 

 producing taste; and others by vapors in the form of gases, 

 causing the sense of smell. Figure 96 shows the microscopic 

 structure of some of these sensory end organs. In each case 

 the end organ is started into activity by an external stimulus, 

 and when thus excited an impulse starts from it over the nerve 

 fiber and passes to the central part of the nervous system. In 

 the central system, the stimulus produces what we call a sensa- 

 tion, and this gives the brain a knowledge of what is going on 

 at the outer end of the nerve. Sensation never occurs until the 

 impulse reaches the brain. From these sensations the brain 

 obtains information as to what is going on in different parts of 

 the body, and upon this information, bases its knowledge and 

 regulates the activities of the body. 



Reflexes. The nervous system is made up of a mass of 

 neurons whose connections with each other are inconceivably 

 complex. These neurons, with their long axons, unite in har- 

 monious activity the different organs of the body, and they do 

 this by virtue of the fact that their axons, though distributed 

 all over the body, all converge in the central system, where they 

 can be associated together by the numerous neuron bodies that 

 compose these central ganglia; Fig. 85. The courses taken by 



