GENERAL PHYSIOLOGY OF NERVE-TISSUE 



9 1 



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 

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

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

 logic significance of Nissl's granules is unknown. The nerve fibrils are 

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

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



W/HLCHM ... 



...HYltJN SWATH. 



imilKAL BMNCH. 



FIG. 43. A. EFFERENT NEURON; B, AFFERENT NEURON. FOUND IN BOTH SPINAL AND CRANIAL 



NERVES. 



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- 

 drites present short oclateral presses, known as lateral buds, or gemmules, 

 which impart to the branches a feathery appearance. This character- 

 istic is common to the cells of the cortex of the cerebrum and of the 

 cerebellum. The ultimate branches of the dendrites, though forming an 

 intricate feltwork, never anastomose with one another nor unite with 

 dendrites of adjoining cells. According to the number of axons, nerve-cells 

 are classified as monaxonic, diaxonic, polyaxonic. Most of the cells of 

 the nerve system of the higher vertebrates are monaxonic. In the ganglia 

 of the posterior or dorsal roots of the spinal and cranial nerves, however, 

 they are diaxonic. In this situation the axons, emerging from opposite 

 poles of the cell, either remain separate and pursue opposite directions, or 

 unite to form a common stem, which subsequently divides into two branches, 

 which then pursue opposite directions. (See Fig. 43, B.) The nerve-cell 

 maintains its own nutrition, and presides over that of the dendrites and the 



