MEDULLATED AND NON-MEDULLATED NERVES. 269 



radiating striae like those which one sometimes meets 

 with upon a small scale in dogs, and pretty constantly in 

 rabbits in different directions. The microscopical exa- 

 mination showed that, like as in these animals, medullated 

 fibres had developed themselves in the retina, and that 

 its fibrous layer had become thicker and opaque in con- 

 sequence of the assumption of medullary substance. On 

 examining the individual fibres I found, on tracing them 

 from the fore and middle parts of the retina backwards 

 towards the papilla, that they gradually increased in 

 breadth, and at the same time displayed, at first in an 

 almost imperceptible, but afterwards in a very striking 

 manner, an investing layer of medulla. This is a kind 

 of transformation, therefore, which essentially impairs 

 the functions of the retina, for this delicate membrane 

 becomes thereby more and more impervious to light, in- 

 asmuch as the white substance does not suffer the rays 

 of light to pass through. 



The same change occurs in nerves during their deve- 

 lopment. A young nerve is a delicate, tubular structure, 

 provided with nuclei at certain intervals and containing 

 a pale grey substance. The medulla does not appear 

 until afterwards, and then the nerve becomes broader 

 and the axis-cylinder becomes distinctly defined. It may 

 be said therefore that the medullary sheath is not an 

 absolutely necessary constituent of a nerve, but is added 

 to it only when it has arrived at a certain stage in its 

 development. 



Hence it follows that this substance, which was for- 

 merly regarded as the essential constituent of a nerve, 

 according to present views plays a subordinate part. 

 Those only who do not even now admit the existence of 

 the axis-cylinder, regard the white substance of course 

 not only as the greatly predominating constituent, but 

 also as the really active element of the nerve-contents. 



