346 MANUAL OF HISTOLOGY. 



beginning of decomposition, which occurs very quickly, the 

 inner half appears as if filled with a finely granular substance, 

 while the outer exhibits transverse striations, and finally 

 breaks up into small disks, which can only be distinguished 

 from those of the outer segment of the cones by their red color 

 (visual purple of Kuhne), which soon fades on exposure to 

 light. 



The inner segment of the cones is larger than that of the 

 rods ; it tapers rapidly toward the outer part, where it is filled 

 with a peculiar oval-shaped body ; the outer segment does not 

 equal that of the rods in height, but divides into similar disks. 



The pigment-layer, in which the ends of the rods and cones 

 are imbedded, consists of a single layer of hexagonal cells, 

 more densely pigmented in the part next the retina, and by 

 some observers said to be provided with fine processes, which 

 are lodged between the rods and cones. This pigment is more 

 dense at the macula and varies with the color of the person, 

 being most abundant in negroes, whereas it is absent in albi- 

 nos ; from this layer, according to Kuhne, the visual purple 

 of the rods is reproduced. 



At the macula lutea, which is situated a little to the outer 

 side of the entrance of the optic nerve, the ganglion-cell and 

 inner nuclear layers have their greatest thickness. The fibres 

 which pass from the outer granular to the outer nuclear layer 

 are lengthened and run in a more horizontal direction toward 

 ihefovea, which forms a slight depression in the centre of the 

 macula. 



Over this fovea the layers of nerve-fibres and ganglion-cells 

 are absent, and the other laminae become so much thinned 

 that the membrana limitans interna approaches nearly to the 

 nuclear layer; the rods are also absent, and the cones be- 

 come lengthened and slightly convergent. 



The meiribrana limitans interna lies between the retina and 

 vitreous body ; it is a transparent homogeneous structure, and 

 from its outer surface spring the connective-tissue fibres which 

 form the supporting framework for the nervous part of the 

 retina. 



These fibres arise in the form of thin fenestrated plates, 

 connected together by numerous arms ; they soon contract, 

 however, to smaller radiating bands, which surround the gan- 

 glion-cells and pass on to the inner nuclear layer, where they 



