664 DISSECTION OF THE EYE. 



fresh, so that an image can be seen on it when the two external coats of 

 the eye have been removed ; but it soon loses this translucency, and is 

 moreover rendered opaque by the action of water and other substances. 

 Its thickness is greater at the posterior than the anterior part of the eye- 

 ball, being J^th of an inch in the former, and T 5o tn ^ n tne latter situa- 

 tion. 



On the outer surface of the dissected retina are some fine fragments of 

 a structure (Jacob's membrane) to be noticed presently, which float in the 



fluid in which the preparation may be placed. 

 Fig. 233. The inner surface is smooth ; it is covered 



with folds in a preparation of an eye cut in 

 two, but these are accidental, in consequence 

 of the membrane having lost its proper sup- 

 port. At the spot where the optic nerve ex- 

 pands (porus opticus) is the central artery of 

 the retina (fig. 233). 



In the anterior of the human eye, in the 

 axis of the ball, is a slightly elliptical yellow 

 part (fig. 233), T ^th of an inch in diameter, 

 which is named the yellow spot (limbus luteus 



o* THB i SURFACE of Sommerring). Almost in the centre of that 

 OF THE RETINA, in the centre S P* ls a minute hollow, the Jovca. centralis, 

 of the ball is the yellow limbus which appears black in consequence of the 

 luteus, here represented by thinness of the wall allowing the dark pigment 



shading ; and in its middle the outs jj e to be seen 



dark spot. To the inner side is . ,f. c\r> ^ i ^i_ j.- 



the nerve, wi ,h its accompany- , Structure (tig. 234). In the retina are 

 ing artery. (Sommerrmg.) layers or strata with bloodvessels, viz., an in- 



ner ( 3 ), composed of nerve elements; an outer 



(') (Jacob's membrane), formed of peculiar bodies; and an intermediate 

 or granular layer ( 2 ); and outside all is a pigmentary stratum. Passing 

 through the layers of the retina except Jacob's and the pigmentary, are 

 minute threads fibres of Miiller. 



The layer of nerve substance ( 3 ) is made up of nerve fibres, and of a 

 molecular matrix containing different-sized nerve cells ; these constituents 

 have the following arrangement : 



The tubules of the optic nerve (/<), having become solid in texture and 

 gray in color from the absence of the white substance of Schwann, radiate 

 in bundles from the end of the optic trunk, and communicate together to 

 construct a thin web at the inner aspect of the nervous layer; this delicate 

 network with lengthened meshes diminishes in strength as it is followed 

 forwards. 



Outside the nerve fibres is a stratum of molecular material (/) with 

 large pale pyriform and roundish nucleated nerve cells (ff) with offsets ; 

 this layer begins at the entrance of the optic nerve, and becomes thinner 

 as it extends forwards. Around the optic nerve the cells are arranged in 

 a layer one deep, but over the yellow spot they are about six or eight 

 deep ; near the ora serrata they are scattered in clusters. An offset from 

 each cell is supposed to join internally the nerve fibres ; and other offsets 

 are prolonged externally into the molecular material (/) and seem lost in 

 it. 



The outer or columnar stratum (') (Jacob's membrane) consists of two 

 different elements rods and cones, which are arranged with their ends 

 inwards and outwards. 



