ANATOMY OF THE NERVOUS SYSTEM 137 



receptor and the other having a 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 (Fig. 546). 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 the 

 axon-like process which connects with the receptor. 



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 

 (Fig. 54C): 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. 



The nervous system as a whole is made up of neurons of these 

 three types. The sensory neurons, as stated above, lead from the 

 receptors to the center; motor neurons lead from the center to the 

 active tissues; and association neurons form the connecting links 

 wherever such are necessary. All sensory neurons communicate 

 with other neurons at their central terminations. Since the central 

 axons are branched (see above) each sensory neuron has a number 

 of such connections. All motor neurons have likewise connections 

 with other neurons at their central ends. Association neurons 

 connect with other neurons at both ends, as they must if they are 

 to serve as links in a chain whose ends are sensory and motor 

 neurons. 



Synapses. Communication between neuron and neuron is 

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

 motor neurons end in masses of fine branches known as end arbori- 



