176 HISTOLOGY 



They frequently branch once or many times, sometimes forming a very 

 dense and extensive network. One process or its multiple parts brings 

 the impulse into the cell and is called the afferent process or dendrite. 

 The impulse passes through (or by) the cell body and is carried away 

 to its destination by the other process, which is called the efferent process 

 or neurite. The direction of the impulse in the nerve cell is invariably 

 the same, from perceptory end to discharging end, and it is never reversed. 

 The nature of this impulse is not known. Its time reactions and other 

 experiments show that it is not specifically electrical. It is not the stim- 

 ulus itself carried through the cell, but a reaction of the cell to the stimulus. 

 The impulse can be controlled and elaborated by the cell and may be 

 retarded or suppressed or repeated in rhythmic order or even accu- 

 mulated and augmented. We are thus unable to arrive at any conclu- 

 sion as yet concerning the exact physical and chemical conditions that 

 underlie the operation of impulse conduction. It is the dendrite that is 

 usually multiple, especially in the motor cells and communicatory nerve 

 cells that receive the stimulation from other nerve cells. It is commonly 

 single in the perceptory or sense nerve cells. 



The cell body is usually large and distinct. It is possessed of all the 

 ordinary cell-organs necessary for its trophic maintenance and in many 

 cases is multinuclear. 



The nerve cells have been more changed in their positions in the body, 

 perhaps, than any other tissue. These changes can also be traced better 

 and serve to explain many features of form and function which would 

 otherwise remain unsolved. The nerve tissues originated phylogeneti- 

 cally on the surface of the body and were primarily ectodermal in char- 

 acter. T.his would be a logical assumption even without further evidence, 

 because it was only such cells as were on the outside of the body that were 

 in contact with changing conditions which they must perceive and to 

 which they must adapt themselves. The primitive nerve cell was prob- 

 ably a perceptory cell with weak powers of conduction and stimulation, 

 which two latter powers must always follow the first. 



From this superficial position all nerve cells but those that must be 

 on the outside (or near enough to it to be accessible to the stimuli) have 

 retreated into the most inner and best-protected positions possible. This 

 is seen to advantage in the ontogeny of nearly all of the higher forms. 

 The most primitive manner of retreat is for the cell body to grow down 

 from the periphery and leave its perceptory process and end-organ at the 

 surface. When the primitive nerve cells became differentiated to perform 

 the three nerve functions specifically, many of the cells moved inside and 

 took their stimuli from the perceptory cells that were still situated at the 

 surface. The inner cells also acquired, by differentiation, new powers 

 which have resulted in the wonderful nervous systems of many animals, 



