PHYSIOLOGY OF THE NERVOUS SYSTEM 307 



are found outside of the body and are known as external 

 stimuli. 



Action of External Stimuli. In the arrangement of the 

 nervous system the most favorable conditions are provided 

 for the reception of external stimuli. Not only do vast 

 numbers of neurons terminate at the surface of the body, 1 

 but they connect there with delicate structures, called sense 

 organs. The purpose of the sense organs is to sensitize 

 (make sensitive) the terminations of the neurons. This 

 they do by supplying special structures through which the 

 stimuli can act to the best advantage upon the nerve end- 

 ings. Moreover, there are different kinds of sense organs, 

 and these cause the neurons to be sensitive to different 

 kinds of stimuli. Acting through the sense organs adapted 

 for receiving them, light, sound, heat, cold, and odors all 

 act as stimuli for starting impulses. Indeed, the arrange- 

 ment is so complete that the nervous system is subjected 

 to the action of external stimuli in some form practically 

 all the time. The work of the sense organs is further con- 

 sidered in Chapters XX, XXI, and XXII. 



How External Stimuli act on Internal Organs. For 

 stimulating the neurons not connected with the body sur- 

 face we are dependent, so far as known, upon the nervous 

 impulses. An impulse started by the external stimulus 

 goes only so far as its neuron extends. But it serves as a 

 stimulus for the neuron with which the first connects and 

 starts an impulse in this connecting neuron, the point of 

 stimulation being where the fiber terminations of the first 

 neuron make connection with the dendrites of the second. 

 This impulse in turn stimulates the next neuron, and so on, 

 producing a series of impulses along a given nerve path. 



i The surface of the body includes the linings of the air passages, food canal, 

 and certain cavities, as well as the external covering or skin. 



