THE EYE. 745 



the retinal fibres are not composed solely of nerve-medulla, for, if they 

 are treated never so thoroughly with ether, they always remain, smal- 

 ler indeed, but more distinct and more opaque than previously. Fibres 

 which have been thus treated enlarge again in cold acetic acid, and dis- 

 solve in alkalies, and consequently consist, perhaps without doubt, chiefly 

 of a nitrogenous substance. 



As respects the course of the nerve-fibres in the retina, this much is 

 certain, that they radiate on all sides from the colliculus nervi optici 

 and constitute a continuous membranous expansion, which extends as 

 far as the ora serrata retina?, and presents any considerable interrup- 

 tion only in the situation of the macula lutea. In this true nervous 

 membrane the fibres are associated into larger and smaller compressed 

 bundles, usually 0*01-0-012 of a line wide, which either mutually anas- 

 tomose at very acute angles, or run for considerable distances parallel 

 with each other. Notwithstanding all that has been stated by various 

 authors, it may be boldly asserted that the terminations of these nerves 

 are as yet wholly unknown ; and, as more will be said upon this subject 

 afterwards, I shall here merely remark that, in any case, they exist not 

 only in front, but in every part of the retina, because the layer of nerve- 

 fibres becomes visibly thicker from before to behind. I have estimated 

 its thickness, in Man, at the bottom of the eye at 0*036, two lines 

 beyond the yellow spot at 0-006-0-008, and near the ora serrata at 

 0-002 of a line. 



5. The limitary membrane, membrana limitans* (b), is a delicate 

 membrane, 0-0005 of a line thick, intimately united with the rest of the 

 retina, which, when that structure is teazed out, and on the application 

 of reagents, is frequently detached in large shreds, and then appears 

 perfectly structureless. On its inner aspect, towards the hyaloid mem- 

 brane (a), when the retina is folded, flattened cell-nuclei are occasionally 

 perceptible, which certainly cannot be referred to an epithelium, and 

 scarcely to the vitreous body, as the latter is always readily separable 

 from the retina. It seems to be different with regard to a clear, light 

 yellowish border, 0*002-0-003 of a line wide, situated on the outer side 

 of the membrana limitans, which, in folds of a perfectly fresh retina, 

 appears, as it were, to be completely blended with the limitary mem- 

 brane, but occasionally exhibits, more or less distinctly, the contours of 

 excessively clear and transparent spherical bodies (fy, 0-0020*003 of a 

 line in size. At a longer interval after death, as well as on the addi- 

 tion of water, a large number of transparent globules, like drops of 

 albumen, are afforded by the clear border of the retina, which then dis- 

 appears altogether, except the membrana limitans, frequently also, 



" the tendency to fall into the varicose or beaded state ; in a word, that the fibres of the 

 nerve, in expanding into the retina, lose their white substance, but retain the axis- or cen- 

 tral fibre." -TBS.] 



* [So named by Pacini (1. c., p. 22). -TR3.] 



