OF THE NERVOUS SYSTEM. 403 



portions, which are everywhere visible in ordinary microscopic prepa- 

 rations of the brain substance, are termed " myeline drops," and owe 

 their peculiar appearance to the nature of the ingredients which form 

 the medullary layer of the nerve fibre. The medullary layer is com- 

 posed of a substance termed myeline, which is not, however, a distinct 

 proximate principle, but is itself a mixture of various different mate- 

 rials. It consists mainly of cerebrine, a nitrogenous matter found only 

 in the nervous centres, together with a large proportion of cholesterine 

 and fat. There is also a certain proportion of lecithine, a nitrogenous 

 and phosphorized matter, which is also found in the gray substance. 

 The mixture of these ingredients gives to the myeline its peculiar con- 

 sistency and reaction. 



In regard to its physiological function, the medullary layer of the 

 nerve fibre is generally considered as an isolating substance, like the 

 gutta-percha envelope of a submarine telegraph wire, so arranged as to 

 confine the transmission of nerve force within proper limits, and prevent 

 its diffusion to neighboring parts. We have no absolute proof that 

 such is its true character, but there are some facts which lend a certain 

 probability to this view. The medullary layer exists throughout the 

 main portion of a large majority of the nerve fibres, where they trans- 

 port the nervous stimulus uninterruptedly from one point to another ; 

 but they are destitute of it both at their origins and terminations, 

 where they come in contact with the elements of the gray matter, or are 

 connected with the peripheral organs of sensation and motion. What- 

 ever may be its exact function, therefore, the medulla evidently plays a 

 secondary, and not a principal part, in the physiological action of the 

 nerve fibre. 



The Axis Cylinder The central part of the nerve fibre consists of a 

 pale, homogeneous, or finely granular cord, of a cylindrical or slightly 

 flattened form, occupying the position of the longitudinal axis of the 

 fibre. From these characters it has received the name of the " axis 

 cylinder." It differs from the medullary layer, by which it is enveloped, 

 in consistency ; for while the latter is nearly fluid in its natural condi- 

 tion, the axis cylinder is solid, and, though very delicate, possesses a 

 certain degree of elasticity. By some observers (Schultze, Gerlach) the 

 axis cylinder is regarded as composed of many excessively minute fibril- 

 Ise, united into a uniform bundle; by others of equal authority (K61- 

 liker) the indications of such a fibrillated constitution of this part of the 

 nerve fibre are considered as uncertain. 



The axis cylinder is composed of an albuminous substance which is 

 insoluble in water, alcohol, and ether ; becomes pale and swollen by the 

 action of concentrated acetic acid ; and is readily dissolved by a boiling 

 solution of sodium hydrate. It is stained red by treatment with a solu- 

 tion of carmine, while the enveloping medullary layer remains un- 

 colored; and by this means a visible distinction may be made between 

 the two. The application of a solution of gold chloride, and subsequent 

 exposure to light, stains the axis cylinder of a dark purple, nearly 



