NERVE 53 



transmittino: nerves and of the receiving stations in the 

 central nervous system. 



This involves the study of the motor reactions which 

 may follow any given set of stimuli. 



Before proceeding to study the physiology of the complex 

 set of receptors, nerves, central nervous system, and effectors, 

 it will be well first to consider — 



1. How the nervous system is developed. 



2. The physiology of single nerve units. 



I. Development of the Nervous System. 



To form the nervous mechanism, a part of the epithelial 

 covering of the embryo sinks inwards as a canal, and the 

 cells of this 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 undifferenti- 

 ated and alike (neuroblasts), but later some of them throw out 

 processes (a) towards the surface, and others (6) towards the 

 reacting structures. These cells with their outgrowths form 

 the units of which the nervous system is built up — the 

 Neurons. They are separate from one anotlier, but are 

 associated by the close propinquity of their branching twig- 

 like processes or dendrites, such an association being known 

 as a synapse (fig. 17). 



Fig. 17. — To show a receirinc) (c) and a reacting Neuron (a), each with den- 

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

 a Synapsis [h). 



1. Neurons to and from the Body Wall- — The nerve fibres 

 are formed as outgrowths from the nerve cells. This has 

 been demonstrated by the histological investigations of 

 Ramon-y-Cajal, who used a method of impregnating the 



