Miscellaneous. 479 



gray substance ; 4. the expansion of the optic nerve ; and 5. the 

 limiting membrane. 



Leaving the latter membrane out of the question, we will commence 

 with the layer of the optic nerve. The most remarkable fact to be 

 noticed here, is that according to a discovery of M. KoUiker, the 

 expansion of the optic nerve is interrupted at the macula lutea, so 

 that at that place there does not exist the smallest trace of a layer of 

 nervous fibres. In all other parts of the retina the nervous fibres 

 form a very thick and uninterrupted layer at the bottom of the eye ; 

 except that at the margins of the macula lutea they are lost in a 

 layer of nervous cells, which here form the most internal layer of the 

 retina, and are only covered by the limiting membrane. These cells 

 form a very thick layer in this place, as in a vertical section of the 

 retina, from nine to twelve series of cells placed one behind the other 

 may be seen ; these cells possess the characters of the other cells of 

 the retina, which will be referred to hereafter. 



With respect to one of the most important questions, the termina- 

 tion of the nervous fibres of the retina, the observations of M. Kolliker 

 upon the human retina show that these fibres are in direct commu- 

 nication with the nervous cells. These cells, which are entirely ab- 

 sent at the entrance of the optic nerve, are all provided with from 

 one to six processes, exactly resembling those found in the nervous 

 cells of the brain and ganglions ; these, ramifying several times, are 

 contiimous with the true nervous fibres of the expansion of the optic 

 nerve, so that these fibres take their rise in the nervous cells of the 

 retina. The discovery of this important fact is due to the Marquis 

 A. Corti, of Turin, who ascertained it, first, about three years ago in 

 the ruminants and afterwards in the elephant, in which animal the 

 origins of the optic fibres are presented with unparalleled distinctness 

 and beauty. We have verified the facts ascertained by M. Corti on 

 the human retina, and think we may say that in man, as in the other 

 mammalia, there are terminations of the nervous fibres in the cells of 

 the retina. As to free terminations, we have never found them, and 

 we are inclined to believe that, although admitted by several authors, 

 they have no existence in fact. 



Next to the nervous fibres and cells, the parts most worthy our 

 attention are the cylinders and cones. The former have been well 

 described by Hannover ; but the cones of the retina of man and 

 mammalia have not been sufficiently seen by any observer. The 

 cones are pyriform or conical bodies, three or four times the thickness 

 of the cylinders, but not half their length, situated in the in- 

 terior of the layer of cylinders. The cones, which on their outer 

 slender portion bear a prolongation resembling a short cylinder, are 

 less numerous than the cylinders ; they are tolerably regular in their 

 arrangement. At the position of the macula lutea, as observed by 

 Henle, there are no cylinders, whilst the cones are very numerous and 

 form an uninterrupted layer. 



One of the facts most worthy attention is, that from the internal 

 portion of each cone and from each cylinder a fibre proceeds, which, 

 after traversing all the layers of the retina, loses itself on the inner 



