THE SPINAL CORD AND BRAIN 



813 



((/) Cells with monopolar dendrites. Several main dendritic stems spring from one pole of 

 the cell and, undergoing frequent subdivision, break up into a fine terminal arborization. The 

 axone springs from the opposite pole (Purkinjean .cells of the cerebellum; granular cells of the 

 fasciola cinerea). 



2. Nerve Cell Body. Internal Morphology. The nucleus of the nerve cell differs in no essen- 

 tial from the typic nuclear structure. Regarding the organization of the cytoplasm several 

 conflicting views exist. In the present state of our knowledge 

 concerning this still obscure field of investigation it may be 

 said that the nerve cell protoplasm is roughly divided into 

 a peripheral exoplasmic portion and a central endoplasmic 

 portion. There is shown throuyhotit the cytoplasm a tendency 

 ro fibrilliir structure, more pronounced in the exoplasmic 

 portion. Within the meshes of a more or less homogeneous 

 ground substance, which pervades the whole, are deposited 

 larger and smaller masses of a granular substance. Nerve 

 cells fixed and stained by the methods of Nissl and Held 

 show that the granule masses are "stainable" (chromatophils ; 

 tigroid bodies; Nissl bodies), probably of the nature of a 

 Ducleoproteid (MacCallum) and looked upon as a sort of 

 nutritive reserve. Many of the larger cells possess more 

 or less pigmented material, adjacent to the nucleus. The 

 cells of the substantia nigra (intercalatwri) and of the 

 locus coeruleus contain an abundance of such pigment 

 granules. 



The " unstainable" homogeneous ground substance of the 

 Cytoplasm is probably the more important functionally, for 

 numerous delicate neurofibrils have, by special methods, been shown to traverse the cell body 

 and its processes, crossing and interlacing, perhaps anastomosing with each other, and traceable 

 into the axone. 1 Nissl, after years of painstaking investigation, has classified nerve cells into 

 a great many different species in accordance with their reaction to staining agents. 



A. rone 



FIG. 584. Motor nerve cell from 

 ventral horn of spinal cord of rabbit. 

 The angular and spindle-shaped Nissl 

 bodies are well shown. (After Nissl.) 



Axone. 





Sheath of 

 cell body. 



Nucleus. 



Cell protoplasm. 



iffy ~ Atone. 

 />' > Myelln sheath. 



FIG. 585. Bipolar nerve cell from a spinal ganglion FIG. 586. Three stages in the development of a cell 



of the pike. (After Kolliker.) from a spinal ganglion. 



The Dendrites. The dendrites are attenuated processes, usually numerous, resembling 

 in structure and staining reactions the cytoplasm, of which, as extensions, they increase the 

 functional expanse of the surface of the cell body. Emerging by a broad base, they become 



1 That the neurofibrils form such an intracellular network and that the axones arise therefrom is disputed 

 by Ramon y Cajal, Bielschowsky, and others. 



