THE CENTRAL NERVOUS SYSTEM 



Varieties of Neurons. 

 Nearly all the nerve- 

 cells of the cerebro- 

 spinal axis agree with 

 the cells of the anterior 

 horn in the possession of 

 an axon and one or more 

 dendrites, although 

 sometimes the dendrites 

 are scanty in number 

 and insignificant in size. 

 In the cerebral cortex 

 the typical cells are of 

 pyramidal shape. From 

 the base comes off the 

 axon, and from the an- 

 gles dendritic processes, 

 a particularly massive 

 dendrite proceeding 

 from the apex of the 

 pyramid towards the 

 surface of the brain. 



Sometimes an axon, 

 instead of ending in an 

 arborization which 

 comes into relation with 

 the dendrites of another 

 nerve-cell, or, as is more 

 frequently the case, with 

 the dendrites of more 

 than one cell, breaks up 

 into a sort of basket- 

 work of fibrils surround- 

 ing the cell-body. The 

 cells of Purkinje, for 

 instance, in the cere- 

 bellum are surrounded 

 by such pericellular bas- 

 kets (Fig. 308). The 

 cells of the spinal gan- 

 glia have two axons, 

 which in the embryo 

 arise one from each end 

 of the bipolar cell, but 

 in the adult, in all verte- 

 brates except some 

 fishes, are connected to 

 the cell by a single pro- 

 cess (Fig. 306). The 

 great majority of them 

 have no dendrites, un- 

 less, as some have sup- 

 posed, the peripheral 

 process really represents 

 a dendrite. Another 

 kind of cell which seems 

 undoubtedly to be of 



FIG. 307. SCHEME OF LOWER MOTOR NEURON 

 (BARKER). 



a, h, axon-hillock (the portion of the cell from 

 which the axon comes off), containing no Nissl 

 bodies, and showing fibrillation ; ax, axis-cylinder 

 or axon ; m, medullary sheath, outside of which 

 is the neurilemma ; c, cell-substance (cytoplasm), 

 showing Nissl bodies in a lighter ground substance ; 

 d, protoplasmic processes or dendrites containing 

 Nissl bodies ; n, nucleus ; n', nucleolus ; n, R, 

 node of Ranvier ; s, f, side fibril ; n of n, nucleus 

 of the neurilemma ; tel., motor end plate ; m', 

 striped muscle-fibre ; s, L, incisure. 



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