828 



THE CENTRAL NERVOUS SYSTEM 



insignificant in size. In the cerebral cortex the typical cells are of 

 pyramidal shape. From the base comes off the axon, and from the 

 angles 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 surrounding the cell- 

 body. The cells of Purkinje, for instance, in the cerebellum are sur- 

 rounded by such pericellular baskets (Fig. 326). The cells of the spinal 

 ganglia have two axons, which in the embryo arise one from each end 

 of the bipolar cell, but in the adult, in all vertebrates except some 

 fishes, are connected to the cell by a single process (Fig. 324). It has 

 been commonly held that the unipolar cell with a single T-shaped 



process is developed from a bipolar cell, 

 which grows towards one side, so that the 

 two processes come together and fuse. 

 Such observations as that of Harrison on 

 the bifurcation of the growing end of the 

 main process of isolated nerve-cells culti- 

 vated in vitro suggest an alternative and 

 a simpler explanation viz., that the 

 T-shaped process is derived from the 

 splitting of a single chief process. If this 

 be the case, one of the original processes 

 at the poles probably undergoes a retarded 

 development or disappears, since the great 

 majority of the spinal ganglion cells with 

 the T-shaped process appear to have no 

 dendrites. Another kind of cell which 

 seems undoubtedly to be of nervous nature 

 is the ' granule-cell.' Granule-cells are 

 much smaller than the nerve-cells we have 

 been describing. Their processes are much 

 less easily followed, but all appear to give 

 off an axon and several dendrites. They 

 contain a relatively large nucleus (5 to 8 /* 

 in diameter), with only a mere fringe of 

 cell-substance. The nucleus, unlike that 

 of a large nerve-cell, stains deeply with 

 haematoxylin. Some parts of the grey 

 matter are crowded with these granule-cells e.g., the nuclear layer 

 of the cerebellum and the substantia gelatinosa, or substance of 

 Rolando, which caps the posterior horn in the cord. In other parts 

 they are more thinly scattered, but probably they are as widely diffused 

 as the large nerve-cells proper, and no extensive area of the grey matter 

 is wholly without them. 



Although there are several varieties of granules (Hill), they all 

 agree in this, that their axons run a comparatively short course, and 

 never, or rarely, pass beyond the grey matter. Another kind of neuron 

 which is also confined to the grey matter, and is typically seen in the 

 cortex of the cerebrum and cerebellum, presents the peculiarity of an 

 axon which branches into an intricate network immediately after 

 coming off from the cell (cell of Golgi's second type). Unlike the long 

 axon of the typical large nerve-cell, the axis-cylinder process of this 

 Golgi cell remains unmedullated. 



The sympathetic ganglion cells are developed from immature neuro- 



Fig. 326. Pericellular Baskets 

 (Schafer, after Cajal). Two 

 cells of Purkinje from the 

 cerebellum are seen sur- 

 rounded by end ramifications 

 forming a basket-work, b, de- 

 rived from the branching of 

 axons of small nerve-cells in 

 the molecular layer; a, axon. 



