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TEXT-BOOK OF PHYSIOLOGY. 



having a diameter of not more than 10 to 12 micro-millimeters, the largest 

 not more than 150 micro-millimeters. Each cell consists of granular, 

 striated cytoplasm, containing a distinct vesicular nucleus and a well-de- 

 fined nucleolus. A characteristic feature of the cytoplasm is the presence 

 of granules first described by Nissl, which stain deeply with methylene blue 

 and other dyes. For this reason these granules are spoken of as chromo- 

 phile granules. The remainder of the cytoplasm is penetrated in various 



AXON 



A. B. 



FIG. 44. A. EFFERENT NEURON; B. AFFERENT NEURON. 



directions with nerve fibrils which are continuous with similar fibrils run- 

 ning through the axonic process as well as the dendrites. The physiolo- 

 gic significance of Nissl's granules is unknown. The nerve fibrils are prob- 

 ably connected with the transmission of nerve impulses. A cell mem- 

 brane has not been observed. From the surface of the adult cell portions 

 of the cytoplasm are projected in various directions, which portions, 

 rapidly dividing and subdividing, form a series of branches, termed den- 

 drites or dendrons. In some situations the ultimate branches of the den- 



