698 A MANUAL OF PHYSIOLOGY 



is a sensory path, bifurcation of the axon occurs, one branch passing 

 down along the previous motor path to its proper muscular termina- 

 tion, and the other passing down the sensory path. Although there 

 is no evidence that efferent fibres can unite with afferent fibres, 

 a degenerated afferent path can therefore serve as a chemiotactic 

 scaffolding or guide for the growth of regenerating motor axons, 

 though not such an efficient one as a degenerated motor path. 

 Sensory fibres, however, cannot regenerate along motor paths or 

 make functional union with the receptive substance of skeletal 

 muscle. 



It is a remarkable fact that regeneration of the fibres of the central 

 nervous system either does not in general occur, or is exceedingly 

 difficult to realize. This lends support to the doctrine of the 

 importance of the neurilemma in regeneration, since the neurilemma 

 is absent from the fibres of the brain and cord. It has, however, 

 been shown that regeneration of the fibres which proceed from the 

 cells of the spinal ganglia along the posterior roots into the cord may 

 take place after the roots have been cut, so that the normal reflexes 

 through the respiratory, cardiac, and vaso-motor centres may be 

 once more obtained. 



Degeneration of Muscle. Experimental section or, in man, 

 traumatic division or compression of a nerve leads not only to 

 its degeneration, but ultimately, if regeneration of the nerve 

 does not take place, to degeneration of the muscles supplied by 

 it as well. The muscle-fibres dwindle to a quarter of their normal 

 diameter ; the stripes disappear ; the longitudinal fibrillation 

 fades out ; and at length only hyaline moulds of the fibres are 

 left, filled, and separated by fatty granules and globules and 

 surrounded by engorged capillaries. Amidst the general decay, 

 the muscular fibres of the terminal ' spindles ' with which the 

 afferent nerves of muscles are connected, alone remain un- 

 changed (Sherrington) . Certain diseases of the cord which 

 interfere with the cells of the anterior horn cause degeneration 

 of motor nerves, and ultimately of muscles. The motor nerve- 

 endings degenerate sooner than the sensory. Both may, under 

 suitable conditions, regenerate (Huber). 



Reaction of Degeneration. Muscles whose motor nerves have 

 been separated from their trophic centres show, when a certain 

 stage in degeneration has been reached, a peculiar behaviour to 

 electrical stimulation, called the ' reaction of degeneration.' To 

 the constant current the muscles are more excitable, and the con- 

 traction slower and more prolonged than normal. When a current, 

 either constant or induced, is passed through a normal muscle, 

 the muscular fibres may be stimulated either directly, or indirectly 

 through the intramuscular nerves. Under ordinary conditions the 

 nerves respond more readily than the muscular fibres, especially to 

 momentary stimuli like induction shocks, and therefore the so-called 

 direct stimulation of uncurarized muscle is, as a rule, an indirect 

 stimulation. When the muscle is curarized and the nerves thus 

 eliminated, the excitability to induced currents is found to be 

 diminished. The same is the case in a muscle which exhibits the 



