NEKVE-CELLS. 



507 



Axon 



j found in the rest of the nervous system. They differ not only in their mode of 



prigin and in their subsequent development, but also in the connexions of their 



! nerve-fibre processes. 



Nerve-cells of the Brain and Spinal Medulla. The cells in the cerebro- 



1 spinal axis are variable both in size and form. Some are relatively large, as, for 

 example, certain of the pyramidal cells of the cerebral cortex and the motor cells 



[in the spinal medulla, which almost come within the range of unaided vision; 



1 others are exceedingly minute, and require a high power of the microscope to bring 

 them into view. The cell consists of a protoplasmic nucleated body, from which 



>the axon proceeds, and the protoplasmic processes of Deiters, or the dendrites 



j Tig. 447). 



The axon presents a uniform diameter and a smooth and even outline. It gives 

 off in its course fine 

 collateral branches, 

 but does not suffer 

 thereby any marked 

 diminution in its 

 girth. The most 

 important point to 

 note in connexion 



with the axon, how- flE '%. frreaggHlfc-. _ + 



ever, is the fact that 

 it becomes continu- 

 ous with the axis- 

 cylinder of a nerve - 

 fibre. The axon 

 then is simply a 

 nerve-fibre, and in 

 certain circum- 

 stances it assumes 

 one or two invest- 

 ing sheaths, of which 

 more will be said 

 later. The axon 

 may run its entire 

 course within the 

 substance of the 

 brain or spinal 



: medulla, either for a 

 short or a long dis- 

 tance (intercalated 

 cells), or it may 

 emerge from the 

 brain or spinal 



: medulla in one of 

 the cerebral or spinal 



! nerves as the essential part of an efferent nerve-fibre, and run a variable distance 



: before it finally reaches the peripheral* structure in relation to which it ends 

 (efferent nerve-cells). The axon and the collaterals which spring from it appear to 

 terminate either in small button-like swellings or knobs, or more frequently in 

 ; terminal arborisations, the extremities of which seem to be furnished with ex- 

 ceedingly small terminal varicosities. In those cases where the axon or its 

 ^laterals end within the brain or spinal medulla, some of the terminal arborisa- 

 tions interlace with the dendrites of nerve-cells, whilst others are twined around 

 the bodies of other cells. In the latter case the interlacement may be so close 

 and complete that it almost presents the appearance of an enclosing basket-work. 

 In cases where the axon emerges from the cerebro-spinal axis its terminal arborisa- 

 tion ends in relation to a muscle-fibre or some other tissue in the manner described 

 below. 



FIG. 447. THREE NERVE-CELLS FROM THE ANTERIOR COLUMN OF GRAY 

 MATTER OF THE HUMAN SPINAL MEDULLA. 



