118 THE PHYSIOLOGY OF NERVE 



development of a current of injury. At this time a gradual retro- 

 gression sets in which terminates eventually in a complete loss of irri- 

 tability.^ The interval of time required for the development of these 

 changes varies in accordance with the type of the animal, the con- 

 dition of the nervous tissue and the severity of the lesion. In warm- 

 blooded animals, for example, the excitability is lost in from 2 to 4 

 days, while in cold-blooded animals it generally takes a much longer 

 time. For the sciatic nerve of the frog this interval is usually given 

 as 33 days, although it may be as long as 3 or 4 months. Evidently, 

 this difference is dependent upon the nutritive condition of the animal 

 and the temperature, because the degeneration sets in much sooner 

 during the summer and frequently progresses at this time with a speed 

 equal to that observed in the mammals. In young and vigorous 

 animals its progress is more rapid. It should be remembered, how- 

 ever, that the development of these changes necessitates the complete 

 separation of the fibers from the cell-body, because if they are merely 

 divided and their ends left in contact with one another, the degenera- 

 tion is prone to assume an abortive character. The irritability then 

 fails to decrease and besides, the morphological changes do not develop 

 with any degree of definiteness. 



Degeneration is classified as primary, secondary, and tertiary. 

 The primary type involves the nerve-fibers at the seat of the injury 

 and affects solely those internodal segments which have been directly 

 exposed to the trauma. Beginning at this point, the degeneration 

 first progresses outward in the direction of the conduction of these 

 axis-cylinders until it finally involves their distalmost branches. 

 This marked implication of their peripheral stumps constitutes sec- 

 ondary degeneration. As far as the efferent paths are concerned, it may 

 be inferred that their destruction must render the effector functionally 

 useless, because its separation from the cell shuts out those central dis- 

 charges which normally keep it in tonus and activate it. Thus, while 

 the degeneration of nerve really ceases in the end-plate, it also impli- 

 cates in an indirect way the tissue with which it is in functional rela- 

 tion. The latter then suffers a disarrangement of its metabolism in 

 consequence of the loss of the usual efferent impulses. Thus it may be 

 observed that the destruction of a musculomotor nerve is invariably 

 followed by atrophic changes in the muscle innervated by it. It is 

 noted that the muscle fibers decrease in thickness, and that their cross- 

 sections lose their sharp contours and fibrillar appearance. They 

 eventually assume a hyaline appearance and become widely separated 

 from one another by infiltrated fat. Very similar changes result in 

 afferent paths. The direction of the degeneration in them may be 

 either centrifugal or centripetal in accordance with the location of the 

 cell-bodies. 



In either case the destruction of the conducting path must lead to 

 an isolation of the cell-body and its dendrites, thereby rendering the 

 1 Waller, Miiller's Archiv, 1852, 392. 



