560 YELLOW SPOT OF THE RETINA. [SECT. 226. 



ternal to the yellow spot, coob'" to o , oo8'" ; and near the ora 

 serrata, o , oo2 / ". 



5. The limitary membrane, membrana limitans (fig. 224, 8), is 

 a delicate membrane, o'ooo5"' at the most in breadth, intimately 

 united with the rest of the retina ; it sometimes, however, becomes 

 detached in large shreds on teasing out the retina, and on the 

 employment of re-agents, and then it is seen to be perfectly 

 structureless. It resists acids and alkalies for a long time, and 

 resembles in other respects also the so-called vitreous membranes, 

 such as the capsule of the lens. 



The yellow spot is an elliptical part of the retina, 1 •44'" long, 

 0*36"' broad, of a yellow or golden-yellow colour, whose inner 

 extremity is distant ro'" to ri'" from the centre of the elevated 

 circle formed by the entrance of the optic nerve. It possesses 

 almost in the middle, but somewhat nearer to the inner end, an 

 attenuated, colourless depressed spot, o - o8'" to o - i"' in diameter. 

 The fold, as it is called, which many authors assume to exist on 

 the yellow-coloured part, and have named plica centralis retina, 

 has been shown by Virchoic and myself, as well as by others (our 

 observations were on an executed criminal), not to be present in 

 fresh eyes; but the yellow colour is seen in such cases, and is 

 produced by a diffused pigment saturating all the parts of the 

 retina, with the exception of the bacillar layer ; the colour fades 

 in a few days after keeping in alcohol and water. With reference 

 to the structure of the yellow spot, it is wanting in a continuous 

 layer of nerve-fibres, the superficial layer especially being absent : 

 the nerve-cells lie in several strata immediately upon the mem- 

 brana limitans, being placed close together like the cells of a 

 pavement epithelium. Between these cells nerve-fibres run, others 

 also entering from the circumference of the macula, and they 

 are lost in them in a manner not yet accurately determined, 

 probably in the nerve-cells. The finely granular layer of grey 

 nervous substance (^Pacini's ' fibre griggie'), is found on the outer 

 parts of the yellow spot, but is absent in the middle. The two 

 granular layers, and the intermediate granular lamina, occur over 

 the whole of the macula, the fovea centralis excepted. The rods, 

 as Henle discovered, and as I can confirm, are absent over the whole 

 yellow spot, and are replaced by closely disposed cones, which are 

 longer and narrower (o"002 w ) than elsewhere, and bear a slender 

 rod upon their outer side. I have observed filaments of the cones, 

 or fibres of Midler, in all parts of the yellow spot, with the excep- 

 tion of the fovea centralis ; I have not made any particular in- 

 vestigation of them in respect of their terminations, but they 



