400 SPECIAL HISTOLOGY. 



of tubes of less recent nerve substance, all of which, as innumera- 

 ble examples of isolated axis-fibres occurring in them, show, 

 invariably contain such fibres. The axis- cylinder, possessing the 

 same refractive power as the still fluid part of the medullary 

 sheath, is necessarily indistinguishable from it, but from this 

 circumstance we cannot conclude that it is absent, nor, equally, 

 can such a conclusion be drawn from its invisibility in the 

 recent nerve-fibril. Taking all these circumstances together, I 

 am firmly convinced that a special, central structure exists even 

 in recent nerves, which is distinguished from the more ex- 

 ternal portion, — that is, from the medullary sheath, — not only 

 by its chemical composition, as appears to me to have been 

 placed beyond all doubt, but also by its consistence and 

 elasticity, as well as by its possessing a determinate form. The 

 condition in which we obtain the axis-fibre in the human nerves 

 and central organs, by the addition of the serum of the blood, 

 albumen, or vitreous humour, appears to me to represent its 

 natural state ; on the other hand alcohol, ether, iodine, corrosive 

 sublimate, gallic-, and chromic acid render it more consis- 

 tent than it is normally; whilst acetic acid, dilute nitric 

 acid, and alkalies exhibit it paler and more swollen. The 

 nerve-pulp forms a semi-fluid cortex around the axis-fibre, 

 and, though intimately connected, is not continuous with 

 it. By pressure, therefore, the pulp may very frequently be 

 expressed, by itself, from the ends of the tubes or from lateral 

 rents of the sheath. The drops of pulp thus formed, usually 

 coagulate on the surface, remaining clear and transparent in the 

 interior, like the central portion of the nerve-tubes. Many 

 authors have described these bodies as portions of the whole 

 contents of the nerve-tube, and have regarded their formation 

 as a proof against the pre-existence of the axis-fibre, but 

 incorrectly. They belong to the medullary sheath only, which, 

 in the interior of all nerve-fibres with only a double contour, 

 is still for some space perfectly clear and bright. An axis- 

 fibre and a clear- space, in fibres having a double contour, are 

 therefore by no means identical, and it is not at all surprising, 

 nor opposed to the existence of an axis-cylinder, that a multitude 

 of drops with a double contour and clear contents should be 

 obtained from such fibres. The medullary sheath may also 

 coagulate entirely, and then the axis-fibre remains evident, 



