STRUCTURE OF THE RETINA. 615 



8. The pigmentary layer. — This layer, which bounds the retina 

 externally, and was formerly known as the hexagonal pigment of the 

 choroid, consists of a single stratum of hexagonal epithelium cells 

 (fig. 425). The outer surface of each cell — that which is tm'ned 

 towards the choroid — is smooth and flattened, and the part of the cell 

 near this surface is devoid of pigment, and usually contains the 

 nucleus ; the inner boundary, on the other hand, is not well marked, for 

 the substance of the cell, which here is loaded with pigment, is pro- 

 longed into excessively fine, straight, filamentous processes (fig. 425, h), 

 v^'hich extend for a certain distance between and amongst the outer 

 segments of the rods and cones. The pigment granules are placed 

 for the most part, both in the cells and cell-processes, with their 

 long axes at right angles to the surface of the retina. The intervals 

 between the rods and cones are only partially filled by the ]3rocesses of 

 the hexagonal pigment-cells ; the remaining part appears to be occu- 

 pied by a clear pellucid material which, according to Henle and H. Midler, 

 is of a soft elastic consistence during life and in the fresh condition, 

 but soon liquefies after death ; but according to Schwalbe, would appear 

 to be normally liquid. In the embryo, between the hexagonal pigment 

 and the remainder of the retina, there is a distinct cavity or cleft filled 

 with fluid (remains of primary optic vesicle). 



The sustentacular tissue of the retina : Mtillerian or radial 

 fibres. — In addition to the elements above described, which are for the 

 most part special to the particular layer where they occur, there are 

 certain other structures which are common to all the layers, passing 

 through the whole thickness of the retina from the inner almost to the 

 outer surface, and, if not actually themselves of the nature of connec- 

 tive tissue, at least serving the same kind of purpose, namel}', to bind 

 together and support the more essential and delicate structures of the 

 membrane. These sustentacular fibres or fibres of Midler, commence 

 at the inner surface of the retina by a broad conical base or foot, which 

 is not solid but hollowed out, the hollow being filled by granular pro- 

 toplasmic substance, and often containing a nucleus. The feet of 

 adjoining fibres are united together at their edges, so as to give, in 

 vertical sections of the retina, the appearance of a distinct boundary 

 line (fig. 419) ; this has been named memhrana limitans interna, but, as 

 may be inferred from the above description, it is in no way a continuous 

 or independent membrane. The fibres pass through the nerve- and 

 ganglionic layers, with a smooth contour, or with but two or three well- 

 marked lateral projections from which fine lamellar processes extend 

 amongst the elements of these layers : gradually diminishing in size 

 they then traverse the inner molecular layer, without, according to 

 Schwalbe, becoming actually connected with the soft substance which 

 mainly composes it, although in the mammalian retina the fibres are 

 marked by slight projections in passing through this layer. In the 

 inner nuclear layer they again give off excessively delicate flattened 

 jDrocesses from their sides, which pass around the inner granules and 

 serve to support them. Moreover, each Mtillerian fibre is here charac- 

 terised by the presence of a clear oval or elliptical nucleus (already 

 mentioned in the description of the inner nuclear layer), containing a 

 distinct nucleolus, and situated at one side of, and in close apposition 

 to the fibre to which it belongs. On reaching the outer nuclear layer 

 (after passing through the outer molecular) the fibres of Miiller break 



