92 TEXT-BOOK OF PHYSIOLOGY 



axon as well. If the latter be separated in any part of its course from the 

 cell, it speedily degenerates and dies. 



The Axon. The axon, or nerve process, arises from a cone-shaped pro- 

 jection from the surface of the cell, and is the first outgrowth from its cyto- 

 plasm. At a short distance from its origin it becomes markedly differentiated 

 from the dendrites which subsequently develop. It is characterized by a sharp, 

 regular outline, a uniform diameter, and a hyalin appearance. In structure, 

 the axon appears to consist of fine fibrillae 'embedded in a clear, semi-fluid 

 material, the neuroplasm. The axon varies in length from a few millimeters 

 to one meter. In the former instance the axon, at a short distance from 

 its origin, divides into a number of branches, which form an intricate felt- 

 work 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 brain and spinal cord, it gives off a number 

 of collateral branches, which possess all its histologic features. The long 

 axons serve to bring the body of the cell into direct relation with peripheral 

 organs, or with more or less remote portions of the nerve system, thus con- 

 stituting association or commissural fibers. Physiologic investigations have 

 established the fact that the axon is the conducting agent of the nerve 

 impulses. 



The Myelin. At a short distance from the cell the more or less elon- 

 gated axon becomes invested with nucleated oblong cells, which subse- 

 quently become modified and constitute the medullary or myelin sheath. 

 When fresh the myelin is clear and semi-fluid; when treated with various 

 reagents it becomes opaque and imparts a white appearance to nerves. 

 The function of the myelin is unknown. All axons that possess a myelin 

 investment are known as myelinated nerve-fibers. 



The Neurilemma. The myelin in many situations is enclosed by a 

 thin transparent elastic membrane known as the neurilemma. In the 

 spinal cord and brain, the nerve fibers are for the most part wanting in 

 this membrane. 



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 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. Beneath the neurilemma in each 

 internodal segment there is a large nucleus surrounded by a small amount of 

 granular protoplasm. 



The End Tufts. 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 myelin 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 histologic and physiologic connection with their cellular elements. In 



