THE NERVOUS SYSTEM 



169 



and fatigue. Although the different sensations must be re- 

 garded as dependent on the integrity of the brain, they arise 

 from some other part of the body. Lack of space forbids a 

 discussion of each sensation, but those of hearing, sight, and 

 touch will be discussed in Chapter IX. It is important to under- 

 stand that all sensations are made possible by a perfectly ar- 

 ranged sense mechanism. 



A sense mechanism is composed of a sense-organ, or sensory 

 nerve-ending, adapted to receive; a path of afferent and efferent 

 nerve-fibers adapted to conduct; either an area in the brain, or 

 a nerve-center in the spinal cord, adapted to interpret. 



^^-Hb — • 



A.N. 



Fig. 55. — Diagram to show the simplest possible basis for a reflex action: A.N., 

 afferent nerve; E.N.C., efferent nerve cells; E.N. four efferent nerves. Four 

 synapses are shown at the points where the dendrites of the afferent and efferent 

 nerves connect. 



A voluntary action of the simplest kind is produced in response 

 to the will or emotions. For its consumation there must be an 

 adequate external physical stimulus; conduction of the stimu- 

 lus to the central nervous system; transmission of an impulse to 

 the muscles, which produce the end effect. 



A reflex action is one produced independently of the will in 

 response to an external stimulus. It is always of a distinctly 

 purposeful character, and is made possible by the junction of 



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