THE NERVOUS SYSTEM OF VERTEBRATES 



301 



or corrosive sublimate and stained with methylene blue or toluidine blue, 

 the protoplasm is seen to contain angular masses which are deeply coloured 

 with the dye (Fig. 143). These masses are known as the Nissl granules or 

 bodies. By other methods it is possible to demonstrate that the whole 

 protoplasm of the cell between the Nissl bodies is pervaded by fine fibrils, 

 which enter the cell from the processes and may run out of the cell by the 

 axon or may run into some of the other shorter processes (Fig. 146). The 

 processes of the cell, as is evident from their development, are of two kinds. 

 The axon which becomes continuous with the axis cvlinder of the medullated 



FIGS. 145 and 146. Nerve cells from spinal cord. (BETHE.) 



Fig. 145, showing Golgi network, and neurofibrils : d, e, f, junctions of axons 



with Golgi network. Fig. 146, showing neurofibrils and Nissl bodies. 



nerve fibre arises from a part of the cell body known as the axon hillock, 

 which is the only part of the cell free from Nissl bodies (Fig. 144). The 

 other processes, which may be very numerous, are known as the dendrites. 

 They are generally thicker than the axon at their origin from the cell, but 

 rapidly diminish in size as they give off branches, the branches apparently 

 terminating freely in the grey matter in the immediate neighbourhood of 

 the cell. In specimens stained by the Golgi method the dendrites may 

 sometimes present a somewhat serrated outline. The Nissl bodies of the 

 cell extend some way into the dendrites. 



A nerve cell with all its processes, axon, and dendrites is spoken of as a 

 neuron. From the development of the central nervous system in vertebrates, 

 it is evident that the nervous path of every reaction must be made up of 

 two or more neurons. If we take, for example, the simplest possible reaction 

 which might be effected through a single segment of the spinal cord, we see 

 that the afferent impulse might be started by some stimulus applied to the 

 ramifications in the skin of the distal processes of the posterior root ganglion 

 cell (c/. Fig. 132). The nerve impulse so started is carried by the nerve fibre 



