338 



PHYSIOLOGY 



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. 147). The processes of the cell, as is evident from their 

 development, are of two kinds. The axon which becomes continuous 

 with the axis cylinder of the medullated 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. 145). The other processes, which 

 may be very numerous, are known as the dendrites. They are generally 



FIG. 144. Nerve-cell from the spinal cord, FIG. 145. The point of origin 



stained by Nissl's method. of the axon, the ' nerve- 



a, axis-cylinder process or axon ; 6, proto- hillock,' highly magnified, 



plasm of cell, consisting of c, fibriUated to show absence of Nissl's 



ground substance, and e, the granules of granules from the origin of 



Nissl ; d, nucleus. (LENHOSSEK.) the process. (HELD.) 



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 some- 

 times 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 



