THE RETINA 659 



perfectly fresh it is difficult to make out anything but an entirely homo- 

 geneous substance ; but shortly after death each rod seems to be divided 

 by a delicate line into an outer and an inner segment, the outer being 

 a little the longer. At the upper extremity of the inner segment, is a 

 hemispherical body, with its convexity presenting inward, called the 

 lentiform body. The entire inner segment is somewhat granular and 

 often presents a granular nucleus at its inner extremity. The outer seg- 

 ment apparently differs in its constitution from the inner segment and 

 is not similarly affected by reagents. Treated with dilute acetic acid 

 the outer segment becomes broken up transversely into thin disks. 



The cones probably are of the same constitution as the rods, but the 

 portion called the inner segment is pyriform. The straight portion 

 above (the outer segment) is sometimes called the cone-rod. The entire 

 cones are about half the length of the rods and occupy the inner por- 

 tion of the layer. The outer segment is in its constitution precisely like 

 the outer segment of the rods. The inner segment is slightly granular 

 and contains a nucleus. The cones are connected below with filaments 

 passing into the deeper layers of the retina. 



At the fovea centralis, Jacob's membrane is composed entirely of 

 elongated cones with no rods. These are slightly increased in thick- 

 ness at the macula lutea, but are diminished again in thickness, by 

 about one-half, at the fovea centralis. At the fovea the optic nerve- 

 fibres are wanting ; and the ganglion-cells, which exist in a single layer 

 over other portions of the retina, here present six to eight layers, ex- 

 cept at the very centre, where there are but three layers. Of the 

 layers between the cones and the ganglion-cells, the outer nuclear layer 

 and the outer molecular layer remain in the fovea, while the inner 

 nuclear layer and the inner molecular layer are wanting. At the fovea, 

 indeed, those elements of the retina which may be regarded as purely 

 accessory disappear, leaving only the structures that are concerned 

 directly in the reception of visual impressions. 



The retina becomes progressively thinner from the centre to the 

 periphery. The molecular and granular layers and the nervous layers 

 are lost in the anterior half of the membrane. 



The following is the probable mode of connection between the rods 

 and cones and the ganglion-cells : The filaments from the bases of the 

 rods and cones pass inward, presenting in their course the corpuscles 

 which have been described in the molecular and granular layers, and 

 finally become, as is thought, directly continuous with the poles of the 

 ganglion-cells. The cells send filaments to the layer formed by the 

 expansion of the optic nerve, which are continuous with the nerve- 

 fibres. 



