74 



VETERINARY PHYSIOLOGY 



ism the muscles, glands, etc. To form this, a part of the 

 epithelial covering of the embryo sinks inwards as a canal 

 composed of the surface cells, and these cells form functional 

 connections with the surface on the one hand and with the 

 reacting structures on the other. At first the cells composing 

 this tube are undifferentiated and alike, but later some of them 

 throw out processes towards the surface and others towards 



i wr 



FIG. 35. To show a receiving (c) and a reacting Neuron (a), each with dendrites 

 at its extremities, and their connection to one another through a 

 Synapsis (6). 



the reacting structures, and these are connected, not by actual 

 continuity, but by coming in close relationship to one another 

 in a series of branching processes, forming a synapsis (fig. 35). 



Each of the units so formed has been called a neuron ; and a 

 neuron may be defined as one of the cells with all its processes 

 which build up the nervous system. These neurons may be 



i 



FIG. 36. (a) A Nerve Cell with Nissl's granules ; (b) a similar cell 

 showing changes on section of its axon. 



divided into the receiving and reacting series, but in structure 

 they are alike. 



The shape and characters of the cells, and their position upon 

 the processes of the neuron the fibres vary greatly, but 

 they have all the following characters in common : They are 

 nucleated protoplasts, the protoplasm of which shows a well- 

 marked network, in the meshes of which a material which 



