84 



ANATOMY FOR NURSES. 



[Chap. VII. 



cranial nerves, except that in these either one root or the other 

 is often entirely lacking. 



The relations of the roots, fibres, and so forth, can be best 

 understood from a study of the accompanying diagrams (Figs. 

 71, 72). 



Degeneration and regeneration of nerves. — Since, as has been stated 

 in Chapter 1., the nucleus is essential for the nutrition of the whole cell, it 

 follows that if the processes of a neurone are cut off, they will suffer from 

 malnutrition and die. If, for instance, a spinal nerve be cut, all the periph- 

 eral part will die, since the fibres composing it have been cut off from 

 their cell-bodies situated in the cord, or in the spinal ganglia. The divided 

 ends of a nerve that has been cut across readily reunite by cicatricial 

 tissue, — that is to say, the connective tissue framework unites, — but the 



S.E. 



Fig. 72. — Diaoram showing Relation of Neurones composing the Spinal 

 Nerve-koots with Adjacent Nervous Structures. S.E., seusory epithelium 

 connected by a sensory neurone with spinal cord ; ^'.3/., striated muscle receiving the 

 axons from a motor-cell in the ventral horn of the gray matter in the cord ; Sp. F., 

 spinal fibres, medullat«d, sensory, and the motor, pas.sing to the sympathetic gan- 

 glion where they connect with the sympathetic neurones; S.F., S.F., non-meduUated 

 fibres from the sympathetic neurones passing to the viscera, the axoues going to the 

 plain muscle {P.M.), the dendrones to the sensory endings (S.E.). 



cut ends of the fibres themselves do not unite. On the contrary, the periph- 

 eral or severed portion of the nerve begins to degenerate, the medullary 

 sheath breaks up into a mass of fatty molecules and is gradually absorbed, 

 and finally the axone also disappears. In regeneration, the new fibres grow 

 afre.sh from the axones of the central end of the severed nerve-trunk, and 

 penetrating into the peripheral end of the trunk, grow along this as the 

 axone of the new nerve, each axone becoming after a time surrounded with 

 a medullary sheath. Restoration of function in the nerve may not occur 

 for several months, during which time it is presumed the new nerve-fibres are 

 slowly finding their way along the course of those which have been destroyed. 



