CHAP, in.] SIGHT. 25 



Eesting upon the hind pigment epithelium, lying between it 

 and the vascular layer, is a thin layer about which there has been 

 much dispute. Seen en face the layer appears to consist of a 

 number of long oval or even rod-shaped nuclei, arranged radially 

 in a bed of material which is homogeneous save for some deposit 

 of pigment and an obscure radiate striation. By some authors the 

 nuclei are regarded as the nuclei of plain muscular fibres whose 

 bodies form together the more or less homogeneous bed, and these 

 authors accordingly speak of the layer as a radiate muscle, the 

 dilatator iridis, the contractions of which would tend to widen 

 the pupil. Other authors deny the muscular nature of this layer, 

 and regard it as either a specially developed basement membrane, 

 a continuation of the membrane of Bruch, or as a modified epithelial 

 layer, the continuation of the pigment epithelium of the retina, 

 the hind epithelium of the iris spoken of above corresponding, 

 according to this view, to the pars ciliaris retinae alone. We shall 

 return to this question later on in speaking of the movements of 

 the pupil. 



716. The Cornea. The sclerotic coat towards the front of the 

 eye, at about the level of the attachment of the iris, is somewhat 

 suddenly converted from an opaque membrane into the very 

 transparent body of the cornea. The connective-tissue instead of 

 being arranged in a feltwork by the interlacement of meridional 

 and equatorial bundles as in the sclerotic, is in the cornea arranged /fa. j 

 in parallel, or rather concentric, layers of bundles all placed evenly 

 in the same direction, the bundles of each layer and indeed the ( > 

 fibrillae of the bundles being so united with cement substance that 

 the whole is transparent. Moreover the connective-tissue corpuscles 

 are distributed not irregularly but regularly in single layers 

 between the layers of fibrillated material ; they lie in spaces in the 

 transparent cement substance uniting the layers together. Each 

 cell is a broad flat thin plate with much-branched processes and a 

 large, for the most part, oval nucleus. Since each cell lies with its 

 broad surface parallel to the surface of the cornea and is very 

 thin in the line of the rays of light, and since moreover the cell- 

 substance and the nucleus is, in life, transparent, the presence 

 of these cells does not interfere with the transparency of the 

 organ. 



The front surface of the cornea is covered with an epithelium 

 of the same nature as the epidermis of the skin ( 433) but some- 

 what modified. The cells form a few layers only. The lowermost 

 layer of vertical cells is succeeded by two or three layers of cells 

 corresponding to those of the Malpighian layer, but irregular in 

 form and not bearing prickles. These are in turn covered by cells, 

 two or three deep, which become flattened towards the surface, but 

 retain their nuclei and are not so completely transformed into 

 horny plates as are the corresponding cells of the epidermis. The 

 substance of each cell is sufficiently transparent to render the whole 



