THE NERVE CELL 179 



and auditory), the olfactory (including the gustatory), and the "visual cells 

 and tissues. 



Included in the communicatory cells are those that receive their stim- 

 ulus from another neuron or group of neurons and transmit it as an im- 

 pulse to still other neurons in a chain. Such cells are able to perceive 

 and to stimulate other nerve cells. They exist chiefly to act as transmit- 

 ting units between other neurons. As has been said, some of them can 

 manipulate the impulse in transit, a subject we know but little about. In 

 cases where the nerve-chain is composed of only one neuron, no such 

 specialization has taken place, and the one cell performs all three func- 

 tions. Two neurons in a chain mean that the function of communica- 

 tion is unspecialized. The motor neurons stimulate the muscle cells 

 and other cells into action. This is their chief duty notwithstanding 

 that they must also perceive and conduct. In all but the rarest cases 

 the cell body lies in a ganglion or central ganglion as the anterior horn 

 of the cord in the vertebrates. One exception is formed in some medusae 

 in which the cell body of a neuron with its perceptory organ lies in the 

 periphery and receives stimuli which pass direct to the muscle. Other- 

 wise the cell usually has a number of processes that receive the impulse. 

 These are the dendrites. They may be large and branched as in the 

 electric motor cells of fishes which furnish the stimulus to the electro- 

 plaxes found in these forms. Also in the cerebellum of man when the 

 Purkinje cells have even more branched processes. 



Technic. Very little can be learned about the real structure of nerve 

 tissues from the study of ordinary sections stained in haematoxylin and 

 other ordinary stains. To get any idea of the real disposition of the ele- 

 ments one must resort to a very large number of special and difficult 

 processes that require time and experience for their proper performance. 

 Some of these methods will be mentioned under the several parts of the 

 chapter, but, for the most, the reader is referred to LEE. 



LITERATURE 



JOHNSON, J. B. "Text-book of Neurology," 1906. Saunders & Co., Philadelphia. 

 SCHNEIDER, K. C. Several sections of the "Lehrbuch der vergl. Histologie." Jena, 



G. Fischer, 1902. 

 BARKER, L. "The Nervous System." New York, 1899. D. Appleton & Co. 



THE NERVE CELL 



Although the neuron is probably as highly specialized a cell as there 

 is in the body, still it must execute the ordinary processes of assimilation, 

 of respiration, excretion, etc., which all cells are constantly performing. 

 It has the more of these to do, perhaps, on account of its large size and 



