72 HUMAN PHYSIOLOGY. 



The medulla, myelin, or white substance of Schwann completely 

 fills the neurilemma and closely invests the axis-cylinder. In the 

 recent condition the medulla is clear, homogeneous, semifluid, and 

 highly refracting. In composition it is oleaginous. When the nerve 

 is treated with various reagents which alter its composition, the 

 medulla becomes opaque and imparts to the nerve a white, glistening 

 appearance. The function of the medulla is quite unknown. 



At intervals of about seventy-five times its diameter the medullated 

 nerve-fiber undergoes a remarkable diminution in size, due to an 

 interruption of the medullary substance, so that the neurilemma lies 

 directly on the axis-cylinder. These constrictions, or nodes of 

 Ranvier, taking their name from their discoverer, occur at regular 

 intervals along the course of the nerve, separating it into a series 

 of segments. The portion between the nodes is termed the inter- 

 nodal segment. It has been suggested that in consequence of the 

 absence of the myelin at these nodes, a free exchange of nutritive 

 material and decomposition products can take place between the 

 axis-cylinder and the surrounding plasma. 



The axis-cylinder, or axon, the direct outgrowth of the nerve-cell, 

 is the most essential element of the nerve-fiber, as it alone is uniformly 

 continuous throughout. In the natural condition it is transparent 

 and invisible ; but when ' treated with proper reagents, it presents 

 itself as a pale, granular, flattened band, more or less solid and some- 

 what elastic. It is albuminous in composition. With high magni- 

 fication the axis presents a longitudinal striation, indicating a fibrillar 

 structure. The fibrillse appear to be united by an intervening cement 

 substance. 



Non-medullated Nerve-fibers. These consist, for the most part, 

 only of the axis-cylinder, though in some portions of the nervous 

 system a neurilemma is also present. Though much less abundant 

 than the former variety, they are distributed largely throughout the 

 nervous system, but are particularly abundant in the sympathetic 

 system. Owing to the absence of a medulla, they present a rather 

 pale or grayish appearance. 



Structure of Nerve Trunks. After their emergence from the 

 brain and spinal cord, the nerve-fibers become bound together, by 

 connective tissue, into the form of continuous bundles, which connect 

 the brain and cord with all the remaining structures of the body. The 

 bundles are technically known as nerve trunks or nerves. Each 



