ANATOMY OF THE NERVOUS SYSTEM 117 



tures called neurons, each of which seems to be a single nerve- 

 cell. 



A typical neuron consists of a cell-body containing a nucleus and 

 from whose surface project many rather short branching processes 

 called dendrites, and a single long process having few if any 

 branches and known as the axon. Neurons which convey impulses 

 to muscles (motor neurons) have this structure (d, Fig. 65). 



The neurons which convey impulses from sensory regions to 

 the center (sensory neurons) have a structure which appears at 

 first view, to be altogether different from that of the typical 

 neuron just described. They have cell-bodies with nuclei but in- 

 stead of a single axon and numerous much-branched dendrites the 

 cell-body gives rise to two long axon-like processes, one of which 

 may have a comparatively small number of branches. The bipolar 

 character of these .neurons, moreover, is concealed in many through 

 the union of the two processes for a short distance from the cell- 

 body, giving an appearance as though the latter were on a side 

 branch of a long axon (6, Fig. 65). The underlying similarity of 

 these to the type neuron appears if we consider that the dendrites 

 of the typical neuron are replaced in the sensory neuron by one 

 of the axon-like processes mentioned above. 



A third sort of neurons occurring in the Body resembles the 

 first or motor type in the possession of cell-body and many branch- 

 ing dendrites. Instead of long, slightly branched axons, however, 

 neurons of this sort have short and very much-branched ones. 

 These neurons occur interposed .in the pathway of impulses from 

 sensory to motor neurons and are often called association neurons 

 (c, Fig. 65) : they are not, however, the only sort of association neu- 

 rons; many neurons which belong physiologically to the group of 

 association neurons in that they form communicating paths be- 

 tween sensory and motor neurons are anatomically of the type to 

 which all motor neurons belong. 



If we adopt the usual view that each single neuron represents 

 one nerve-cell, neurons are the largest cells known. Although 

 axons are so small in cross-section as to be microscopic they may 

 have a length of three feet or more, as in the nerve trunks which 

 extend down the legs to the feet. 



Synapses. Communication between neuron and neuron is 

 always according to a certain scheme. The axons of all except 



