CHAPTER II. 



Fundamental Structure of Nervous Organs. 

 Principle of Nerve Components. 



THE nervous system is made up of cells which fall into 

 two categories — those which perform the charac- 

 teristic nervous functions and those which do not. 

 The cells of the latter class, the neuroglia cells, are mainly 

 supportive in function and form a framework in which lie 

 the cells of the former group, the true nerve cells or neurons. 

 There are also strands of ordinary connective tissue extending 

 into the substance of the nervous organs from the membranes 

 which cover them, usually accompanying blood-vessels. 



The neurons vary considerably in form and in structural 

 details, but the typical neuron consists essentially of a cell 

 body or perikaryon, a varying number of branched receptive 

 processes, the dendrites, and a single efferent process, the 

 axon, axis-cylinder, or neurite. The axon frequently has a 

 sheath of fatty material (myelin), the myelin (medullary) 

 sheath, and may give off small branches, usually perpendicular 

 to the main fibre, the collaterals. The nerve cells pass their 

 impulses from one to another through connections known 

 as synapses. The essential structure of the synapse is still 

 the subject of dispute, but it is usually held to be simply a 

 point of close contact, without actual continuity, between 

 the terminal of an axon and the next cell. The details of 

 structure of the nerve cell are described in practically all 

 text-books on the anatomy or physiology of the nervous 

 system and will not be considered here. 



The perikarya of the afferent neurons are nearly always 

 situated outside the central nervous organs, where most of 



