64 HUMAN PHYSIOLOGY. 



reagents, it becomes distinct. Physically, it is quite resistant and elastic. 

 Its function is doubtless that of a protective agent to the structures within. 



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 re- 

 agents 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 dis- 

 coverer, 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 decompo- 

 sition 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 somewhat elastic. It is albuminous in 

 composition. With high magnification 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 nerve is invested by a thick layer of lamellated 

 connective tissue, known as the epineurium. A transverse section of a nerve 

 shows (see Fig. 8) that it is made up of a number of small bundles of fibers, 



