170 AN AMERICA X TEXT-BOOK OF PHYSIOLOGY. 



of which is unknown, but which is supposed to resemble the sarcolemma and 

 to be composed of a substance similar to elastin. The fibres are bound together 

 by connective tissue which on boiling gives gelatin. Within the neurilemma is 

 the medullary sheath, which is composed of two elements — viz. (1) neurokera- 

 tin, a material similar to the horny substance of epithelial structures, which 

 forms a sort of loose trellis, or network, and probably acts as a supporting 

 framework to the fibre; (2) a white, highly refracting, semi-fluid material, 

 which fills the meshes of the neuro-keratin network, and which is composed 

 largely of protagon and cholesterin combined with fatty bodies. Protagon is 

 a complex phosphorized nitrogenous compound, which many observers believe 

 to contain lecithin and cerebrin. According to Noll, it makes up 7.47 per 

 cent, of the dried nerve. Both lecithin and cerebrin are fatty bodies possess- 

 ing nitrogen, and the former phosphorus. These and some other complex 

 fatty bodies probably exist in addition to protagon in the medullary sub- 

 tance. The formation of the "myelin forms" seen in the medulla of dead 

 nerves is attributed to lecithin. The axis-cylinder probably contains most of 

 the proteids of the fibre, chiefly globulins, mixed with complex fatty bodies. 



The reaction of the normal living fibre is neutral or slightly alkaline. It 

 is said to become acid after death, but this change is not known to accompany 

 functional activity. Indeed, nothing is known of the physiological import of 

 the chemical constituents of the nerve-fibre or of the chemical changes which 

 occur in the axis-cylinder when it develops or transmits the nerve impulse. 

 The peculiar chemical composition of the medullary substance would suggest 

 that it has a more important function than simply to protect the axis-cylinder. 

 Some have attributed to it nutritive powers, and others have supposed it helped 

 to insulate: it is certain that the axis-cylinder can develop and transmit the 

 nerve impulse without the aid of the medullary sheath, for there is a large 

 class of important nerves — the non-medullated nerves — in which it is lacking. 



