ADJUSTMENT THE NERVOUS SYSTEM 121 



A reflex is then the physiological unit of the central nervous 

 system, but not the anatomical one. We see that at least two nerve 

 cells are concerned, in addition to the receptor and the muscle, 

 which latter may be called the " effector." These structural units 

 of the nerve centres are known as " neurones," because the use of 

 the name " nerve-cell " was found to lead to confusion. A neurone 

 is a peculiar type of cell in that part of it consists of a fibre, the 

 "nerve fibre," which may be of great length, sometimes several 



FIG. 6. Diagram of the General Arrangements of the Central Nervous 



System. 



A, excitatory association neurone. 



B, E, and H, excitatory afferent (sensory) nerve fibres. 

 C and G, inhibitory afferent nerve fibres. 



D, motor (efferent) neurone, ending in a muscle. 

 F, inhibitory association neurone. 

 The excitatory synapses are white, the inhibitory ones black. 



feet ; but it is, nevertheless, as truly a part of the cell as the pro- 

 truded pseudopodium of an amoeba is. Its structure degenerates, 

 and it loses the power of conduction after it has been cut off from 

 the part of the cell containing the nucleus. This fact enables us to 

 obtain some information as to the nature of the connection between 

 the two or more neurones forming a "reflex arc." If the sensory 

 or "afferent" fibre taking a message to the centre be cut, the 

 portion beyond the place of section degenerates, but only so far as 

 the place where it joins another neurone. The process does not 

 extend beyond the junction, which is called the "synapse"' It is 



