1106 THE ORGANS OF SPECIAL SEXSE 



stained by carmine, iodine, etc., the outer portion remaining unstained with these reagents, but 

 staining yellowish brown with osmic acid. The outer portion of each rod is marked by trans- 

 verse striae, and is made up of a number of thin disks superimposed on one another. It also 

 exhibits faint longitudinal markings. The inner portion of each rod, at its deeper part where 

 it is joined to the outer process of the rod granule, is indistinctly granular; its more superficial 

 part presents a longitudinal striation, being composed of fine, bright, highly refracting fibres. 

 The visual purple, or rhodopsin, is found only in the outer segments of the rods. At its inner 

 end each rod is prolonged into a very fine fibre, the rod fibre, which contains a nucleus, and 

 which terminates in the outer nuclear layer, being somewhat enlarged at its termination. 



The cone cells (Fig. 813) are conical, or flask-shaped, their broad ends resting upon the mem- 

 brana limitans externa, the narrow pointed extremity being turned to the choroid. Each cone 

 cell consists of two parts, the cone and the cone fibre. The outer segment or cone is a short 

 conical process, which, like the outer segment of a rod, presents transverse stria*. The inner 

 segment (cone fibre) resembles the inner portion of the rods in structure, but differs from it in 

 size, bsing shorter and bulged out laterally in a flask-shaped manner, and at the junction of the 

 cone with" the fibre is the nucleus of the cone cell. The cone fibre passes to the outer nuclear 

 layer, and terminates as an expansion from which very minute fibrils are given off. The chem- 

 ical and optical characters of the rod cells and cone cells are identical. 



10. The pigmentary layer or tapetum nigrum, the most external layer of the retina, for- 

 merly regarded as a part of the choroid, consists of a single layer of hexagonal epithelial cells, 

 loaded with pigment granules. Each cell contains a flattened nucleus in the outer portion of 

 the cell which is free from pigment at this point. These cells are smooth externally, where they 

 are in contact with the choroid, but internally they are prolonged into fine, straight processes, 

 whish extend between the rods, this being especially the case when the eye is exposed to light. 

 The pigment changes its position under the influence of light, and is distributed through the 

 entire cell. In the eyes of albinos, the cells of the pigmentary layer are present, but they contain 

 no coloring matter. 



Supporting Framework of the Retina. Almost all these layers of the retina are con- 

 nected by a supporting framework, formed by the supporting cells or supporting fibres of 

 Muller or radiating fibres, from which the membrana limitans internet et externa are derived. 

 These fibres are found stretched between the two limiting layers, "like columns between a floor 

 and a roof," and they pass through all the neural layers except Jacob's membrane. Each com- 

 mences on the inner surface of the retina by a conical hollow base, which sometimes contains a 

 spheroidal body which stains deeply with hematoxylin, the edges of the bases of adjoining 

 fibres being united and thus forming a boundary line, which is the membrana limitans interna. 

 As they pass through the nerve fibre and ganglionic layers they give off few lateral branches; 

 in the inner nuclear layer they give off numerous lateral processes for the support of the inner 

 granules, while in the outer nuclear layer they form a network, the fibre baskets, around the 

 rod and cone fibrils, and unite to form the external limiting membrane at the bases of the rods 

 and cones. In the inner nuclear layer each fibre of Muller presents a clear oval nucleus, which 

 is sometimes situated at the side of, sometimes altogether within, the fibre. The supporting 

 framework of the retina contains neuroglia cells. 



The Path of Light Stimuli. The stimulus is supposed to be first received by the rod and cone 

 cells (the visual cells), and is transmitted to the bipolar cells of the inner nuclear laver and then 

 to the cells of the ganglionic layer, which send fibres by way of the optic nerve and tract to the 

 brain (see p. 911). 



Macula Lutea and Fovea Centralis. The structure of the retina at the yellow spot presents 

 some modifications. In the macula lutea (1) the nerve fibres are wanting as a continuous layer; 

 (2) the ganglionic layer consists of several strata of cells, instead of a single layer; (3) in Jacob's 

 membrane there are no rods, but only cones, and these are longer and narrower than in other 

 parts; and (4) in the outer nuclear layer there are only cone fibres, which are very long and 

 arranged in curved lines. At the fovea centralis the only parts which exist are the cones of 

 Jacob's membrane, the outer nuclear layer, the cone fibres of which are almost horizontal in 

 direction, and an exceedingly thin inner granular layer, the pigmentary layer, which is thicker 

 and its pigment more pronounced than elsewhere. The color of the macula seems to imbue 

 all the layers except Jacob's membrane; it is of a rich yellow, deepest toward the centre, and 

 does not appear to consist of pigment cells, but simply a staining of the constituent parts. 



At the ora serrata (Fig. 802) the nerve layers of the retina terminate abruptly, and the retina 

 is continued onward as a single layer of elongated columnar cells covered by the pigmentary 

 layer. This prolongation is known as the pars ciliaris retinae (Fig. 806), and can be traced for- 

 ward from the ciliary processes on to the back of the iris, where it is termed the pars iridica 

 retinae or uvea. 



From the description given of the nerve elements of the retina it will be seen that there is no 

 direct continuity between the structures which form its different layers except between the 

 ganglionic and nerve fibre layers the majority of the nerve fibres being formed of the axones 

 of the ganglionic cells. In the inner molecular layer the dendrites of the ganglionic layer inter- 



