CENTRAL NERVOUS SYSTEM. 



607 



growth, organization, and rhythms of rest and activity, are more properly 

 presented as functions of all its parts than as functions of special subdivisions. 



PART I. PHYSIOLOGY OF THE NERVE-CELL. 



A. ANATOMICAL CHARACTERISTICS OF THE NERVE-CELL. 



Form of Nerve-cells. Morphologically, the mature nerve-cell is regarded 

 as composed of a cell-body, containing a nucleus together with other modified 

 inclusions and possessed of one or more outgrowths or branches. Some of 

 these branches may be very long, such for instance as those which form nerve- 

 fibres ; other branches are short and differ from the nerve-fibres in their structure. 



The terms employed in describing the nerve-elements are as follows : To 

 the entire mass under the control of a given nucleus and forming both cell- 

 body and branches, the term nerve-cell is applied. The inclusions within the 

 cell -body have the usual designations. Nerve-cells differ greatly in the 

 number of the branches arising from them. In some cells there appear 

 to be two nerve-fibres arising from the cell-body, in others only one. For 

 convenience the description about to be given will apply to the latter group 

 only. From most cells there arises one principal branch, which when con- 

 sidered alone is described as a nerve-fibre, but when considered as the out- 

 growth, of the cell-body from which it originates, is called a neuron. 1 Cells 

 with one neuron are called mononeuric. Cells with two neurons, dineuric. 

 The neuron, in many cases, has branches, both near its origin from the cell- 

 body and also along its course. These branches are designated as collaterals. 

 Contrasted with this principal outgrowth are the other branches of the cell, 



FIG. 143. A group of human nerve-cell bodies, drawn to scale ; X 200 diameters : A, cell-body from 

 the ventral horn of the spinal cord, longitudinal section; C, the same, transverse section; B, cell from 

 the third layer of cerebral cortex ; D, cell from the column of Clarke ; E, cell from the ganglion of the 

 spinal nerve-root, with neuron; F, "solitary" cell from the dorsal horn of the spinal cord ; G, granule 

 from the cortex of the cerebellum (modified from Waller, Human Physiology). 



which are individually much less extensive and which divide dichotomously 

 at frequent intervals. From the tree-like form which they thus acquire they 

 have been designated dendrons. 



The accompanying illustration (Fig. 143) shows the features just described 

 and also gives some idea of the variations in the size of the cell-bodies as found 



1 Schafer, Brain, 1893. 



