44 



ESSENTIALS OF PHYSIOLOGY. 



The only function of these nerve fibres is to conduct impulses. 

 During the propagation of an impulse an electrical current is produced 

 in the fibre, and a current is also induced in it by injury. The excit- 

 ability, conductivity, and electromotivity of the fibre are modified by 

 the passage through it of a constant current. These phenomena have 

 been already described in the chapter on Nerve, and need not be 

 further alluded to in this connection. 



It has already been pointed out that when an axon or other process 

 is separated from the cyton to which it belongs, the severed part 

 undergoes degeneration (fig. 7), usually described as Wallerian 

 degeneration. This may be followed by a growth of fibres from the 



undegenerated portion into the distal 

 part of a divided nerve trunk, the 

 new growth being spoken of as re- 

 generation. 



Degeneration and Regeneration of 

 Nerves. Most nerve fibres contained 

 in a nerve trunk possess two sheaths, 

 (1) a covering of lipoid material known 

 as the myelin sheath, lying next to 

 the fibre and interrupted at regular 

 intervals, and (2) an outer, structure- 

 less covering, the neurolemma. Lying 

 under the latter, about the middle of 

 each myelin segment, is a nucleus sur- 

 rounded by a little protoplasm. 

 When a nerve is divided the part which is cut off from the cell soon 

 dies and undergoes degenerative changes. It shows a temporary rise 

 in irritability followed by a gradual loss of excitability, and after two to 

 five days the nerve is no longer excitable and will not conduct impulses. 

 The degenerative changes take place simultaneously along the entire 

 length of the part of the nerve cut off from the cell, and are visible 

 within twenty-four hours in a warm-blooded animal. The histological 

 changes in a medullated nerve fibre consist first in a thickening of the 

 neurolemma, with enlargement of the nuclei and increase of their 

 surrounding protoplasm, so that the myelin sheath is broken into 

 segments. A few days later the nuclei are seen to have multiplied and 

 become more numerous ; the myelin is in scattered droplets, and the 

 axis cylinder is broken up into short lengths. The myelin and axis 

 cylinder are gradually absorbed by the action of phagocytes, and after 

 three or four weeks nothing is left but the neurolemma containing 

 protoplasmic material in which are embedded many nuclei (fig. 8). 



FIG. 7. Diagram showing effects of 

 section of spinal nerve roots. 



Degenerated portions black, b, section of 

 anterior root ; a, section of posterior 

 root central to ganglion ; c, section of 

 both roots peripheral to ganglion. 



