438 A MANUAL OF VETERINARY PHYSIOLOGY 



the fibre. The cell also suffers in consequence of loss of function, 

 there being no impulses transmitted to it by its dendrites. The 

 neurone concept consists in regarding the nerve-cell, its den- 

 drites, its collaterals, axon, and terminal arborisation as a 

 physiological unit. Innumerable such units, never touching, 

 but yet brought closely together, constitute the nervous system. 



Degeneration and Regeneration of Nerves. — The cell nourishes 

 the axon ; if, therefore, the axon be cut off from its cell, degenera- 

 tion occurs, and this law applies to all cells cut off from their 

 nucleus. We shall see presently that the great inflow and out- 

 flow of nerves from the spinal cord is divided into groups — 

 afferent and efferent — broadly speaking, sensory and motor. 

 Every axis cylinder is a nerve, and has a cell-station. In the 

 case of the sensory nerves the cell-station is the ganglion or its 

 root just outside the spinal canal ; in the case of the motor fibres 

 the cell-station is in the ventral course of the grey matter of the 

 cord. If a spinal sensory nerve be divided below the ganglion, 

 the whole length of nerve below the ganglion, being cut off from 

 its cell-station, degenerates. If the nerve be divided above the 

 ganglion, those fibres which enter the cord from the T division 

 of the two-poled cell alone degenerate, for it is only these which 

 are affected by division above the ganglion (Fig. 131, 1). If the 

 efferent or motor roots be divided, the cell-station being then 

 above the division — viz., in the grey matter of the cord — the 

 nerve degenerates downwards (Fig. 131, 4). If both roots of the 

 spinal nerves be divided below the level of the spinal ganglion, 

 both nerves degenerate in a downward direction (Fig. 131, 3). 

 These profoundly interesting facts, discovered many years ago 

 by the late Dr. Waller, are known after him as Wallerian degenera- 

 tion. By this method of observation it became possible to trace 

 the afferent paths in the spinal cord in consequence of the 

 degeneration produced when these were divided above the 

 ganglion. 



The nerve-fibre, as has been stated above, is but a branch of 

 a nerve-cell. If a portion of a cell be separated from the part 

 containing its nucleus, it soon dies. Thus, when a large amoeba, 

 or a Radiolarian, is torn up into several pieces, the portions 

 containing no nucleus degenerate and die ; but that portion 

 containing the nucleus repairs itself and re-forms a perfect cell. 

 The nerve-fibre dies down after being cut, just in so far as it is 

 a piece of cell cut off from its nucleus. The sensory nerve 

 divided in neurectomy, as practised on the horse, degenerates 

 towards the foot, and not up the limb, for it is the piece below 

 the wound which is cut off from its nutrient centre, and not the 

 portion above. Had this been a motor nerve, the degeneration 

 would still have taken place below the wound, and for the same 



