ADDRESS BEFORE TEE BRITISH ASSOCIATION. 37 



cesses, the axon of the nerve cell, is continued into the axial cylinder of 

 a nerve fiber, and that in the multipolar nerve cell the other processes, 

 or dendrites, branch and ramify for some distance away from the body. 

 A nerve fiber is, therefore, an essential part of the cell with which it is 

 continuous, and the cell, its processes, the nerve fiber and the collaterals 

 which arise from the nerve fiber collectively form a neuron or structural 

 nerve unit (Waldeyer). The nucleated body of the nerve cell is the 

 physiological center of the unit. 



The cell plasm occupies both the body of the nerve cell and its pro- 

 cesses. The intimate structure of the plasm has, by improved methods 

 of observation introduced during the last eight years by Nissl, and con- 

 ducted on similar lines by other investigators, become more definitely 

 understood. It has been ascertained that it possesses two distinct char- 

 acters which imply different structures. One of these stains deeply on 

 the addition of certain dyes, and is named chromophile or chromatic 

 substance; the other, which does not possess a similar property, is the 

 achromatic network. The chromophile is found in the cell body and 

 the dendritic processes, but not in the axon. It occurs in the form of 

 granular particles, which may be scattered throughout the plasm, or 

 aggregated into little heaps which are elongated or fusiform in shape 

 and appear as distinct colored particles or masses. The achromatic 

 network is found in the cell body and the dendrites, and is continued 

 also in the axon, where it forms the axial cylinder of the nerve fiber. 

 It consists apparently of delicate threads or fibrillae, in the meshes of 

 which a homogeneous material, such as is found in cell plasm generally, 

 is contained. In the nerve cells, as in other cells, the plasm is without 

 doubt concerned in the process of cell nutrition. The achromatic fibrillae 

 exercise an important influence on the axon or nerve fiber with which 

 they are continuous, and probably they conduct the nerve impulses 

 which manifest themselves in the form of nerve energy. The dendritic 

 processes of a multipolar nerve cell ramify in close relation with similar 

 processes branching from other cells in the same group. The collaterals 

 and the free end of the axon fiber process branch and ramify in asso- 

 ciation with the body of a nerve cell or of its dendrites. We cannot say 

 that these parts are directly continuous with each other to form an in- 

 tercellular network, but they are apparently in apposition, and through 

 contact exercise influence one on the other in the transmission of nerve 

 impulses. 



There is evidence to show that in the nerve cell the nucleus, as well 

 as the cell plasm, is an effective agent in nutrition. When the cell is 

 functionally active, both the cell body and the nucleus increase in size 

 (Vas, G. Mann, Lugaro); on the other hand, when nerve cells are 

 fatigued through excessive use, the nucleus decreases in size and 

 shrivels; the cell plasm also shrinks, and its colored or chromophile con- 



