GENERAL PHYSIOLOGY OF NERVE-TISSUE 91 



having a diameter of not more thanio 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 



A B - 



FIG 44.-A. EFFERENT NEURON; B, AFFERFNT 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 dendrLs. In some situations the ultimate branches of the den- 

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

 which impart to the branches a feathery appearance his character 

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

 cerebellum. The ultimate branches of the dendntes though formmg an 

 intricate feltwork, never anastomose with one another nor te witn 

 dendrites of adjoining cells. According to the number ' ax f 1, 

 are classified as monaxonic, diaxonic, polyaxomc. Most ot the 

 the nerve system of the higher vertebrates are moronic, 

 of the posterior or dorsal roots of the spinal am 

 they are diaxonic. In this situation the axons, 

 poles of the cell, either remain separate and pursue oW^-~ - _--- - 

 unite to form a common stem, which subsequent^ divides into two branche , 

 which then pursue opposite directions. (See Fig. 44, B. (The nerve cell 

 maintains its own nutrition, and presides over that of t 



