268 



LECTURE XI. 



wherever it is wanting, they appear translucent and grey. 

 There are therefore nerves which are akin in colour to 

 ganglionic matter, are comparatively transparent and 

 possess a more clear and gelatinous appearance than the 

 others ; and they have thence been called grey or gela- 

 tinous nerves (Fig. 78, A). Between the grey and white 

 nerve-substance therefore there does not exist the differ- 

 ence that the one is ganglionic and the other fibrous, 

 but only this, that the one contains medulla and the other 

 does not. In general the absence of medulla in a nerve 

 stamps it as one of a lower and more imperfect kind, 

 whilst the presence of this substance announces a more 

 abundant nutrition and a higher development in the 

 part. 



Not long ago I made an observation in which a direct 

 illustration of the practical importance of these two con- 

 ditions was displayed in a 

 very unexpected manner, the 

 usually translucent grey nerve 

 substance having been trans- 

 formed into an opaque and 

 white matter, namely in the 

 retina. I found namely entire- 

 ly by accident one day, in the 

 eyes of a man in whom I was 

 looking for changes of quite 

 another kind round about 



the papilla of the optic nerve, where the uniformly trans- 

 lucent retina is ordinarily seen a number of whitish, 



Fig. 79. Medullary hypertrophy of the optic nerve within the eye (Of. ' Archiv f. 

 pathologische Anatomic und Physiologic,' vol. x., p. 190). A. The posterior half 

 of the globe of the eye, seen from before ; from the papilla of the optic nerve pro- 

 ceed in four directions radiating striae of white fibres. B. Fibres from this optic 

 nerve in the retina, magnified 300 times : a, a pale, ordinary, slightly varicose fibre, 

 6, one with a gradually thickening medullary sheath, c, a similar one with its axis- 

 cylinder protruding. 



FIG. 79. 



