THE CHEMICAL COMPOSITION OF NERVE-TISSUE 



61 



arises generally from an implantation-cone, but sometimes from one of 

 the other branches (dendrites) close to or at some distance from the cell- 

 body of the neurone. From the neuraxes of the Purkinje cells of the cere- 

 bellum, of the pyramidal cells of the cortex cerebri, of certain cells of the 

 cord, etc., fine branches or collaterals are given off, usually at right angles to 

 the neuraxis. Sometimes the neuraxes are very long(for example, they may 

 extend from the cord to the feet), and much less often they break up soon 

 into arborizations. At the peripheral extremities of all those which do 

 not so divide, there is a tuft of fibrils called teleodendrites. In the course 

 of a nerve the neuraxes seldom branch (save as the fine collaterals), but 

 near their terminations they frequently divide into two, three, or more 

 fibers of a size similar to that of 



the neuraxis. It is part of the FI G- si 



neurone theory that the axis-cylin- 

 der is the centrifugal path i. e., 

 the fiber along which impulses pass 



FIG. 30 



Neuronal terminations in secreting epithe- 

 lium: A, a cell from a rabbit's parotid gland; 

 B, a cell from mammary gland of a cat dur- 

 ing gestation. Observe that the nerve-endings 

 do not connect with the nuclei. (Morat.) 



Perivascular plexus of nerve-fibrils. 

 (Ramon y Cajal.) 



outward from the cell-body. Besides this "centrifugal" branch, most 

 neurones have "centripetal" projections called dendrites. These may 

 be only few in number or very numerous, and often, as in Purkinje's 

 cells of the cerebellum, have arborizations of great extent and complexity. 

 Sometimes the dendrites appear like collaterals, or even like neuraxes, 

 and may then have terminal claw-like teleodendria like those of the latter. 

 In some cases the dendrites are arranged like a basket closely around the 

 nerve-cell of the next neurone in the functional series. But the respective 

 uses of all these structures are not well understood. 



THE CHEMICAL COMPOSITION OF NERVE-TISSUE. 



The facts as to neural composition are clearly of more and more 

 importance as they become better understood. Sooner or later the chem- 

 istry of the nervous system will throw much light on the real nature of its 

 activity, about which so little is now known with certainty. Many things 



