74 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 neighboring 

 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, in- 

 creases 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 con- 

 cerned, 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. (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. 



Union of Nerve Fibers into Nerves or Nerve Trunks. The 

 assembling of nerve fibers into larger or smaller nerve trunks resem- 

 bles histologically the combination of muscle fibers to form a muscle. 

 Physiologically, however, there is no similarity. The various 

 fibers in a muscle act together in a co-ordinated way as a physio- 

 logical unit. On the other hand, the hundreds or thousands of 

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



