THE PHENOMENON OF CONDUCTION. 79 



necessary. Moreover, in the medullated fibers the myehn sheath 

 is lost toward its peripheral end after the nerve has entered the 

 tissue to which it is to be distributed, indicating that its function 

 is then no longer necessary. According to the older conceptions 

 of the process of conduction in nerve fibers, not only anatomical 

 but also physiological continuity is necessary. Mere contact of 

 living axis cylinders would not enable the nerve impulse to pass 

 from one to the other. The newer views, included in the so-called 

 neuron theory, assume that mere contact of living, entirely normal 

 nerve substance does permit an excitatory change to pass from one 

 to the other, so that it is not impossible that the myelin sheath 

 may serve to prevent one axis cylinder from influencing the neigh- 

 boring axis cylinders in a nerve trunk. 



Others have supposed that the myelin sheath serves as a source 

 of nutrition to the inclosed axis cylinder, or as a regulator in some 

 way of its metabolism. No fact is reported that would make this 

 suggestion seem probable. In general, it is found that the myelin 

 sheath is larger in those fibers that have the longest course; the size 

 of the sheath, in fact, increases with that of the axis cylinder. It is 

 known also that the medullated fibers in general are more irritable 

 to artificial stimuh than the non-medullated ones, and that when 

 induction shocks are employed, the non-medullated fibers lose 

 their irritability more rapidly at the point stimulated. None 

 of these facts are suflftcient, however, to indicate the probable 

 function of the myelin. The embryological development of the 

 sheath also fails to throw hght on its physiological significance. 

 For, while it is usually supposed that the axis cylinder itself is 

 simply an outgrowth from the nerve ceU, and the myehn sheath 

 arises from separate mesobiastic cells which surround the axis 

 cyhnder, this view, so far as the myelin is concerned, is not beyond 

 question, and the study of the process of regeneration of nerve 

 fibers inchoates that the actual production of myelin is controlled 

 in some way by the functional axis cylinder. The axis cyhnder 

 outgrowths from the sympathetic nerve cells found in the ganglia 

 of the sympathetic chain and in the peripheral ganglia generally 

 of the body are usually non-medullated, although apparently 

 this is not an invariable rule. In the birds all such fibers are 

 medullated (Langley*). Nothing is known as to the conditions 

 that determine whether a nerve-fiber process shall or shall not be 

 surrounded by a myelin sheath. 



Chemistry of the Nerve Fiber. — Our knowledge of the chem- 

 istry of the nerve fibers is very incomplete. The myelin sheath 

 is composed largely of bodies to which the geseral name of "li- 



* Langley, "Journal of Physiology," 30, 221, 1903; 20, 55, I89S. 



