THE PHENOMENON OF CONDUCTION. 77 



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. 



As some evidence for this view, attention has been called to the fact that 

 in the condition known as multiple or insular sclerosis of the brain and cord 

 the axis cylinders of the areas affected remain intact, while the myelin sheaths 

 are destroyed. The disturbances of co-ordination accompanying this condi- 

 tion may be an expression, therefore, of a loss of isolated conduction. 



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, except, perhaps, the statement that 

 stimulation of a nerve, even for a brief period, causes a change 

 in the appearance of the neurokeratin framework found in the 

 myelin sheath. The change consists in a widening of the meshes 

 (Stiibel*). 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 stimuli 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 sufficient, however, to indicate the probable 

 function of the myelin. The embryological development of the 

 sheath also fails to throw light on its physiological significance. 

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

 simply an outgrowth from the nerve cell, and the myelin sheath 

 arises from separate mesoblastic cells which surround the axis 

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

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

 fibers indicates that the actual production of myelin is controlled 

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

 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, on the 

 contrary, are medullated (Langleyf). 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 general name of " lip- 

 oids " has been applied. This term is used as a generic name for 

 * Stiibel, "Pfliiger's Archiv," 1912, 149, 1. 

 t Langley, "Journal of Physiology," 30, 221, 1903; 20, 55, 1890. 



