PHYSIOLOGY OF NERVE TISSUE. 71 



ture. In its course, however, especially in the central nervous system, 

 it gives off a number of collateral branches, which possess all its 

 histologic features. The long axons seem to bring the body of the 

 cell into direct relation with peripheral organs, or with more or less 

 remote portions of the nervous system, thus constituting association 

 or commissural fibers. 



The more or less elongated axon becomes invested, as a rule, at 

 a short distance from the cell with nucleated oblong cells, which sub- 

 sequently become modified and constitute a medullary or myelin 

 sheath.' This is invested by a thin, cellular membrane the neuri- 

 lemma. These three structures thus constitute what is known as a 

 medullated nerve-fiber. In the central nervous system the outer 

 sheath is frequently absent. In the sympathetic system the myelin 

 is frequently absent, though the axon is inclosed by the neurilemma, 

 thus constituting a non-medullated nerve-fiber. 



The end tufts or terminal organs are formed by the splitting of the 

 axon into a number of filaments, which remain independent of one 

 another and are free from the medullary investment. The histologic 

 peculiarities of the terminal organs vary in different situations, and 

 in many instances are quite complex and characteristic. In peripheral 

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

 the tufts are in direct organic connection with their cellular ele- 

 ments. 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 to- 

 gether 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 the recent condition. When 

 treated with various reagents, it becomes distinct. Physically, it is 

 quite resistant and elastic. Its function is doubtless that of a pro- 

 tective agent to the structures within. 



