CENTRAL NERVOUS SYSTEM. 



643 



variety of incoming impulses to which they are subject. In the case of the 

 afferent, central, and efferent groups of cells, the following can be stated : 

 Unless the branch which passes toward the periphery from the cells of the 

 spinal ganglia be considered as homologous with the deudrons because it car- 

 ries incoming impulses these cells are without dendrons but possess two 

 neurons. In either case they are subject to but one group of impulses those, 

 namely, which enter the cell over the peripheral neuron. The central neuron 

 ramifies widely within the central system. 



Among the central cells we have the greatest variety of arrangement, the 

 dendrons being insignificant in certain cells of the dorsal horns of the gray 

 matter, and abundant in the large pyramidal cells of the cortex ; or again, the 

 granules of the cerebellar cortex with few dendrons may be contrasted with the 

 large cells of Purkinje having many these being taken merely as examples. 



Finally, the bodies of efferent calls are characteristically supplied with a 

 large number of dendrous again an arrangement which fits with the physio- 

 logical demands, as they must react to many stimuli though they discharge 

 but one way and with but one sort of effect. 



Connections between Cells. In determining the connection between 

 cells, the fact that the neuron is the outgrowth of a cell-body and that each 

 cell is an independent morphological unit forms the point of departure. Under 

 these circumstances the question of the connection between cells takes the 

 more explicit form of the question whether cell-branches become continuous 

 by secondary union. In mammals, man included, there is no good histo- 

 logical evidence that such secondary 

 union occurs in the central system. 

 A close approximation of the parts 

 of two nerve-cells is alone to be 

 seen. The means by which the cells 

 are brought close together are not 

 always the same. If the branching 

 of the neurons in the neighborhood of 

 the dendrous of the large pyramidal 

 cells is subject to the interpretation 

 that the impulses act across the small 

 intervals that separate these two struc- 

 tures, then, when it is found that the 

 neurons in some cases end in an en- 

 closing basket or frame about the 

 bodies of the cells of Purkinje, it 

 would be correct to infer that the ac- 

 tion took place between the terminals 

 of the neuron and the body of the cell 

 which they surround. If this infer- 

 ence is correct, then the dendrons are not necessarily the sole pathways for the 

 impulses which affect a given cell (see Fig. 164). 



FIG. 164. Showing at the lower edge of the figure 

 a series of basket-like terminations of neurons 

 which surround the bodies of the great cells of 

 Purkinje in the cortex of the cerebellum (Ram6n 

 y Cajal) : C, cell-body ; N, neurons ; B, basket-like 

 terminations arising from cell C, and enclosing the 

 cells of Purkinje. 



