PHYSIOLOGY OF NERVE TISSUE. 63 



to consist of fine fibrillae embedded in a clear, protoplasmic substance. 

 Shafer advocates the view that the fibrillse are exceedingly fine tubes filled 

 with fluid. The axon varies in length from a few millimeters to 100 cm. In 

 the former instance the axon, at a short distance from its origin, divides into a 

 number of branches, which form an intricate feltwork in the neighborhood of 

 the cell. In the latter instance the axon continues for an indefinite distance 

 as an individual structure. In its course, however, especially in the central 

 nervous system, it gives off a number of collateral branches, which possess alt 

 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 por- 

 tions 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 subsequently be- 

 come modified and constitute a medullary or myelin sheath. This is in- 

 vested by a thin, cellular membrane the neurilemma. These three struc- 

 tures thus constitute what is known as a medullated nerve-fiber. In the 

 central nervous system the outer sheath is frequently absent. In the sympa- 

 thetic 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 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 con- 

 stitute 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 dis- 

 tinct structures.: 



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



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 



