180 THE HUMAN BODY. 



At 1, Fig. 81, is shown a typical nerve-cell such as may 

 be found in an anterior horn of the gray matter of the spinal 

 cord. It consists of the cell body, or cell protoplasm, in 

 which is a large nucleus containing a nucleolus. From the 

 body of the cell arise several branches,, the great majority of 

 which are granular and divide frequently in a forking or 

 " dichotomous " manner. These are known as the " proto- 

 plasmic " branches of the cell, and possibly serve merely to 

 absorb nourishment for it. One branch, however, a, gives oft' 

 at right angles smaller filaments, but still maintains its in- 

 dividuality and ultimately becomes the axis cylinder of a 

 nerve-fibre. Its side branches probably put it in anatomical 

 continuity with other nerve-fibres and other nerve-cells. 

 Nerve cells from the posterior horn of the grey matter of 

 the spinal cord (2, Fig. 81) also possess numerous granular 

 protoplasmic processes and a nerve-fibre process (#); but the 

 latter, instead of continuing directly into an axis cylinder, 

 breaks up into a network of fine branches which frequently 

 unite with one another and also, no doubt, with fibrils from 

 neighboring cells. It is almost certain that one or more of 

 these fibrils or a bunch of them forms the axis cylinder of a 

 fibre in a dorsal root of a spinal nerve. 



As we shall learn later, the dorsal roots are concerned in 

 carrying impulses from the skin and other sensitive parts to 

 the spinal cord; the anterior roots in conveying impulses from 

 the nerve-centres to the organs (muscles, glands, etc.) of the 

 Body. Therefore, in general terms, we may speak of the type 

 of nerve-cell 1, Fig. 81, as a motor nerve-cell; and the type 

 of cell 2, Fig. 81, as a sensory nerve-cell. Both varieties of 

 cells are found abundantly in the gray matter of the brain 

 (Fig. 83), along with other forms, of which the pear-shaped 

 cells of Purkinje existing in the cerebellum may be mentioned 

 (Fig/82). 



In the sympathetic and sporadic ganglia somewhat simpler 

 forms of nerve-cells, having fewer branches, occur. As a rule 

 nerve-cells are comparatively large and have conspicuous 

 nuclei, but in the brain many small ones exist. 



Neuroglia. In the brain and spinal cord the true nervous 

 elements are intertwined with and supported by connective 

 tissue and minute blood-vessels, but in addition there is found 

 closely twisted around the cells and fibres a peculiar tissue 



