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A MANUAL OF PHYSIOLOGY 



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 /z 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 net- 

 work immediately after coming off from 

 the cell (cell of Golgi's second type) . Un- 

 like 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 neuroblasts 

 that migrate, in the course of develop- 

 ment, from the rudiments of the spinal 

 ganglia, and gathering in clumps form 

 the ganglia of the sympathetic chain 

 (His). They agree in general with the 

 cells of the cerebro-spinal axis in pos- 

 sessing an axon and one or more, com- 

 monly several, dendrites, although a 

 few of them are devoid of dendrites. 

 The great majority of the axons remain 

 unmedullated, but a few acquire a very 

 fine medullary sheath. 



The epithelium lining the central 

 canal of the cord and the ventricles of 

 the brain has also been considered by 



some as of nervous nature. The fact that the deep ends of the cells 

 are continued into processes which pierce far into the grey substance 

 has been supposed to lend weight to this opinion, but there is no 

 good ground for it. 



Growth of Neurons. The growth of a neuron is a comparatively 

 slow process. Early in foetal life (about the third or fourth week in 

 man) certain round germinal cells make their appearance amid the 

 columnar ectodermic cells surrounding the neural canal. From their 

 division are formed, in the first months of embryonic life, the primi- 

 tive nerve-cells or neuroblasts. These soon elongate and push out 

 processes, first the axon or axons, and then the dendrites (Fig. 305). 

 As development goes on, the cell-body grows larger, and the processes 



FIG. 308. PERICELLULAR BAS- 

 KETS (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. 



