76 HUMAN PHYSIOLOGY 



special reaction. This is brought about by the establishment 

 of what may be compared to a series of shunting stations 

 between the receptive mechanism on the surface and the 

 reacting mechanism, the muscles, glands, &c. To form this 

 a part of the original covering of the embryo sinks inwards as 

 a canal composed of the surface cells, and these cells remain 

 in functional connection with the surface on the one hand 

 and with the reacting structures on the other. At first the 

 cells forming this tube are undifferentiated and alike, but 

 some throw out processes towards the surface and others 

 towards the reacting structures, and they 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. 34). 



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

 and a neuron may be defined as one of the cells, with all 



. 



Fia. 34. 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). , 



its processes, which build up the narvous $ystem. These 

 neurons may be divided into the deceiving 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 fibre varies 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 stains 

 deeply with basic stains, and which seems to be used up 

 during the activity of the neuron, may accumulate. The 

 granules formed of this material are generally known as 

 Nissl's granules (Fig. 35). 



All these cells give off at least one process, which continues 

 for some distance, as the axon or axis cylinder of a nerve 

 fibre. Frequently other processes are given off, which may 

 either pass away as fibres, or may, while still in close 

 proximity to the cell, form a branching system of dendrites. 



