MINUTE STRUCTURE OF THE NERVOUS TISSUES 31 



it can despatch impulses to other neurons, but they cannot 

 react upon it. What follows is most important : axons are 

 nearly always channels for the conveyance of impulses 

 away from the perikaryon dendrites are receptive in 

 function. It is probably a fact that the majority of neu- 

 rons are of the type having numerous dendrites and only 

 one axon. The arrangement makes stimulation from many 

 directions a possibility, but gives a fixed course to the 

 resulting outflow of energy. Vast numbers of neurons 

 are to be found in the central gray matter which have 

 only short processes and which do not easily admit of 

 distinguishing dendrites from axons, yet the distinction 

 is assumed. 



Fig. 6. To show how in an afferent neuron the transmission is 

 from the endings subject to stimulation (R) to a synapse (S) in the 

 central nervous system. The perikaryon has an intermediate posi- 

 tion and its functions other than that of maintenance are problem- 

 atic. 



One kind of neuron stands somewhat in contrast with 

 all the other varieties, inasmuch as it has a smooth peri- 

 karyon without sign of dendrities. A single axon runs 

 for a short distance away from the perikaryon and is then 

 found to fork and to be traceable for a long way in either 

 direction. This type of unit is peculiar to the afferent 

 department of the nervous system. It is marked physio- 

 logically by the fact that it is not adapted to be stimulated 

 by other neurons, but by energies acting from sources 

 external to the nervous mechanism. In such a neuron 

 the perikaryon is usually buried far from the normal 

 operation of external stimuli. If it bore the ordinary 

 short dendrites and lay open to stimulation within the 



