186 MUSCLE AND NERVE 



Nerve Degeneration and Regeneration. Whether physical or 

 chemical in character, the propagation of a nerve impulse is 

 dependent upon the living condition of the nerve fibers, and 

 this in turn is dependent upon the maintenance of nutritive 

 relations with the nerve cells. Every nerve fiber is the axis 

 cylinder of a nerve cell. If it be cut away from the cell it will 

 degenerate. In a mammal in about four days. A typical degen- 

 eration can only take place in living fibers. The fibers break up 

 into ellipsoidal segments of myelin, each containing a piece of 

 axis cylinder. The segments break up into smaller fragments 

 and undergo absorption. Simultaneously the nuclei of the neu- 

 rilemmal sheath have begun to multiply, and each has formed 

 about itself a mass of protoplasm, so that finally a continuous 

 band of protoplasm, known as an "embryonic fiber" or "band 

 fiber/' has been formed. If, at this time, the connection with 

 the central stump of the nerve is established, the "embryonic 

 fiber" becomes changed into a normal fiber, probably by a 

 growth of the axis cylinder from the central stump. The 

 latter, during the process of degeneration and regeneration, 

 remains practically unaltered. A typical degeneration may 

 extend back for several internodal segments. If, however, no 

 functional union has been reestablished the central cells undergo 

 atrophy in the course of time. Usually distinct histological 

 changes may be observed within the first twenty-four hours in 

 that the chromatin material loses its staining power. The cell 

 becomes swollen and the nucleus assumes an excentric position. 

 These changes progress for eighteen days or so, after winch the 

 cell may regain its normal appearance. 



Myogram. The movements made by striated muscles are 

 too quick to be followed accurately by the eye, so that resource 

 is^had to what is known as the graphic method. The muscle, 

 by means of a mechanism, is made to write its contractions and 

 relaxations on a surface moving at a uniform rate. The entire 

 arrangement constitutes a myograph, and the record thus ob- 

 tained is a myogram. The myogram of a simple muscle contrac- 

 tion consists of three distinct portions. Immediately succeeding 

 the stimulation is an interval (latent period) of about y^ of a 

 second, during which the muscle makes no apparent change. 

 During the next T f^ of a second the muscle shortens, and dur- 

 ing the last y-f-Q of a second it lengthens again. Contraction 



