iz6 TEXT-BOOK OF PHYSIOLOGY. 



many instances are quite complex and characteristic. In peripheral 

 organs, as muscles, glands, blood-vessels, skin, mucous membrane, 

 the tufts are in direct histologic and physiologic connection with their 

 cellular elements. In the central nervous system the tufts are in more 

 or less intimate relation with the dendrites of adjacent neurons. 



Nerve-fibers. The axons with their secondary investments 

 together constitute the nerve-fibers, and according as they possess or 

 do not possess the medullary sheath, they may be divided into two 

 groups viz., medullated and non-medullated fibers. 



Medullated Nerve-fibers. These consist for the most part of 

 three distinct structures: 



1. An external investing sheath, tubular in shape, termed the neuri- 



lemma. 



2. An intermediate semifluid substance the medulla or myelin. 



3. An internal dark thread the axis-cylinder. 



The neurilemma is a thin, transparent, homogeneous membrane 

 closely adherent to the medulla. Owing to its colorless appearance, 

 it can be seen only with difficulty in fresh tissue. When treated 

 with various 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 oj Schwann completely 

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

 fresh tissue 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 a white, glistening appearance. JThe 

 function of the medulla is quite unknown. 



//At intervals of about seventy-five times its diameteiythe medul- 

 la te<T 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 

 Rtmvier. taking their name from their discoverer, occur at Teguiar 

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

 segments. The portion between the nodes is termed the internodal 

 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 uni- 

 formly continuous throughout. In the natural condition it is trans 

 parent and invisible; but when "treated with proper reagents, it 

 presents iisell as a pale, granular, flattened band, more or less solid 

 and somewhat elastic. flit is albuminous in composition. fl With 



