NERVE ELEMENTS AND THEIR FUNCTIONS. 77 



be delivered in full force, strengthened or inhibited, — these things 

 which are all-important in the life of the organism depend upon 

 the form of the nerve elements and the manner in which they 

 are arranged. While the work done by the nervous system is 

 our primary interest, we can understand this only by first under- 

 standing the construction of the mechanism which does the work. 

 On the other hand the v/ork done must be held in mind in order 

 that we may truly interpret the constituent parts of the mechanism. 

 The study of the minute structure and physiology of the nervous 

 systems of various classes of animals has shown that nerve elements 

 usually transmit impulses in a given direction. This is equiva- 

 lent to saying that the elements have a specific form, as otherwise 

 all directions would be alike. In vertebrates as a rule nerve 

 elements present structurally and functionally a polar differen- 

 tiation; the two ends of the element differ in both form and 

 function. The nerve element consists of a mass of protoplasm 

 containing a nucleus, it is a cell. Usually the nucleus is immedi- 

 ately surrounded by a larger mass of protoplasm which is called 

 the cell-body. From this cell-body there extend more or less 

 slender strands of protoplasm called processes. In nearly all 

 cases the processes are seen to be of two forms. Some are rela- 

 tively thick, irregular and have numerous branches; one is rela- 

 tively slender, uniform in diameter, gives off collateral branches 

 and is profusely branched at the end of its course. The former 

 bear a general resemblance to a bush or branch of a tree and are 

 hence called dendrites or dendrons; the latter is more thread like 

 and since it forms the axial and essential portion of a common 

 nerve fiber it is called a neurite or axone. A nerve cell commonly 

 possesses two or more dendrites but usually has only one neurite. 

 The volume of all the dendritic processes may exceed by many 

 times the volume of the cell-body, and even the volume of the 

 neurite may be considerable when it is very long or has many 

 end branches. In certain cases the cell-body is elongated and the 

 dendrites arise from one end and the neurite from the other. Most 

 nerve cells, however, do not so clearly illustrate polar differentia- 

 tion in their form. The dendrites may arise irregularly from 

 various parts of the cell-body and the neurite may take its origin 



