354 SPECIAL HISTOLOGY. 



The only objection which can be urged against this opinion consists in 

 the circumstance, that the axis-fibre cannot be seen in living fresh 

 nerves, and that it cannot generally be distinguished, as a special 

 structure, in the interior of the nerve-tubes without the aid of reagents. 

 But it must be remarked that it can also he brought into view in nerves 

 that are still warm. Thus I find well-marked projecting axis-fibres at 

 the roots of the cerebral nerves in Frogs just killed, which I have 

 examined as quickly as possible, after the application of a solution of 

 sugar, particularly in those of the optic, trigeminal, and vagus, also in 

 the spinal nerves, for instance in the second. I see them under the 

 same conditions in the peripheral nerves of the Frog that have been 

 teased out, and, in these nerves, have on several occasions, even dis- 

 tinctly noticed the axis-fibres in the form of convoluted filaments, in 

 larger drops of the nerve-pulp expressed from the tubes (Fig. 137, 2). 

 The only fact, therefore, that can be adduced in opposition, is this, that 

 it is quite true that the axis-fibre cannot, with certainty, be perceived 

 in the iiiterior of the recent nerve-tubes themselves, except upon the 

 application of some reagent ; but this circumstance obviously proves 

 nothing at all, because neither can it be seen in the interior of tubes of 

 less recent nerve-substance, all of which, as innumerable 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 external 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 humor, appears to me to represent its natural 

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

 gallic, and chromic acid render it more consistent 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, re- 

 maining clear and transparent in the interior, like the central portion 

 of the nerve-tubes. Many authors have described these bodies as por- 



