NERVE ELEMENTS AND THEIR FUNCTIONS. 8l 



As each nerve cell has two poles, an in-coming and an out- 

 going, so the nervous system as a whole receives stimuli from or 

 through other tissues and gives out stimuli to other tissues. The 

 nerve elements are so arranged in the nervous s^ stem that certain 

 elements serve to receive all the stimuK, while others give out 

 impulses to the rest of the body. The former cells may be called 

 receptive cells; the impulses which they carry into the central 

 nervous system, afferent impulses. The cells which give out 

 impulses may be called excitatory cells; and their impulses efferent 

 or out-going impulses. The afferent impulses do not always 

 give rise to sensations. When they do, the impulses may properly 

 be called sensory impulses and the fibers which carry them, sensory 

 fibers. The efferent impulses go to glands as well as to muscles. 

 Those which go to muscles may properly be called motor or excito- 

 motor impulses; those which go to glands may be called excito- 

 glandular impulses; and the fibers concerned may be given corre- 

 sponding names. 



The elements of the nervous system are so arranged that the 

 dendrites of the receptive cells are directed toward the periphery 

 and constitute what are commonly known as sensory nerve fibers. 

 The neurites of the excitatory cells likewise extend to the periphery 

 and constitute excito- motor and excito-glandular fibers. These 

 two sets of fibers, together with numerous sense organs, constitute 

 the peripheral nervous system, within the limits of which the 

 cell-bodies of the receptive cells may also be included. A great 

 number of other cells, which make up the central nervous sys- 

 tem, are engaged in transmitting impulses from the receptive to 

 the excitatory cells, and in distributing and coordinating the im- 

 pulses in such ways as to produce through the excitatory cells 

 definite responses to the stimuH received. The only collective 

 term for all the various categories of cells performing such func- 

 tions is the term central cells. The relations of the sympathetic 

 system to the peripheral and central portions of the nervous sys- 

 tem will be taken up in a special section (Chapter XIII). 



In order to understand the functional relations of the several 

 kinds of nerve cells to one another and to the rest of the organism, 

 a certain type of relatively simple actions which are performed 



