750 



THE NERVOUS SYSTEM 



in the body in that it gives off outgrowths or processes of relatively great length 

 and characteristic structure. The neurone may be defined as a nerve-cell with 

 all its processes, however numerous and far-reaching. As a class of tissue-elements, 

 all neurones possess characteristics which distinguish them from other tissue-elements, 

 but within this class they vary greatly. They vary both according to function and 

 according to their locality in the nervous system. Fig. 555 gives illustrations of 

 the external form of a few of the types found in the human nervous system in 

 the localities stated. 



The cell-body of the neurone gives off two types of processes :- 

 dritic processes or dendrites. These serve to increase the absorbent surface of 

 the cell-body for purposes of nutrition and also act as recipient surfaces for impulses 

 transmitted to the neurone. They are, therefore, cellipetal processes. They branch 

 at frequent intervals and always dichotomously and with rapid decrease in 

 diameter. (2) The axone or neuraxis. Each neurone properly possesses but one 

 of these. In most cases it is very much longer than any of the dendrites, and along 

 its course it maintains a practically uniform diameter. Axones comprise the so- 

 called axis-cylinders of all nerve-fibres, and thus they are the essential components 



FIG. 556. SHOWING PIECES OF AXONES. 



A. From a cerebrospinal nerve. B. From the spinal cord. C. From the sympathetic, a, axones; 

 m, medullary sheath; n, node of Ranvier; s, sheath of Schwann with occasional nuclei. 



of all nerves and nerve fasciculi. Their usual nervous function is to convey impulses 

 away from the cell-body of the neurone, either to other neurones in a chain or to 

 appropriate elements of other tissue systems. Thus axones are cellifugal processes. 

 They branch less frequently than dendrites, and in branching suffer almost no ap- 

 preciable diminution in their diameter. Branches given off along the course of the 

 axone are called collaterals. These are given off at right angles instead of dichot- 

 omously, thus differing from the form of branching of the dendrites. However, at 

 its final termination the axone breaks up into numerous twigs and always dichot- 

 omously. The terminals of these twigs are known as telodendria. Though of 

 minute diameter, the total bulk of the axone is relatively great. The longer of 

 them for example, those extending from the spinal cord for the innervation of the 

 foot may each comprise a volume more than two hundred times that of the cell- 

 body giving origin to it. 



The great majority of the axones of the nervous system acquire a protective and 

 isolating envelope or sheath about them which begins a short distance from the 

 parent cell-body and is lost only upon approaching the twigs of termination. In their 

 study axones are divided according to the character of their sheaths. Those which 

 possess in their sheaths that peculiar mixture of fats called myelin are medullated 



