PRELIMINARY ANATOMICAL AND HISTOLOGICAL OBSERVATIONS. 819 



.concluded that there must be a direct communication between the veins and the 

 anterior chamber, as a diffusion of these substances through membranes is im- 

 possible. 



' Internally to the choroid lies the single layer of hexagonal epithelial cells, from 

 0.0135 to 0.02 mm. in diameter, which are filled with pigment. This layer belongs 

 really to the retina. In front of the ora serrata it forms a double layer of cells, 

 which extends to the posterior surface of the iris (Fig. 266, x). In albinos it is 

 free from pigment; the outer cells on the ridges of the ciliary processes are also 

 devoid of pigment. 



III. Internal tunic of the bulb, consisting of the retina (optic portion) and its con- 

 tinuations, the ciliary and iridic portions of the retina. 



The retina is bounded externally by the hexagonal pigment-epithelium (Fig. 

 268, Ft), which embryologically and functionally belongs to the retina. The cells 

 are not flat, but send pigmented processes into the spaces between the ends of 

 the rods. In some animals the cells contain drops of fat (rabbit) and other sub- 

 stances. At the ora serrata the cells are larger and darker. Of the true layers 

 of the retina: (i) The visual cells, or the "rods" (50 and "cones," called also "neu- 

 roepithelium," lie most externally. They are absent at the optic-nerve entrance. 

 The outer portions of the rods contain, during life, a red pigment, the "visual 

 purple," which is preserved in the dark, but is bleached by daylight, and is con- 

 tinually reprodticed in the eye. It may be extracted by 2.5 per cent, solution 

 of the biliary acids, especially from retinas that have lain in TO per cent, solution 

 of sodium chlorid. The rods are from 0.04 to 0.06 mm. high, and from o. 0016 to 

 o.ooiS mm. broad, and exhibit longitudinal striation, due to depressions; in the 

 axis runs a fine fibril. The outer segment breaks up, occasionally, into numerous, 

 exceedingly fine transverse discs. Krause found an ellipsoid body, the "rod- 

 ellipsoid," at the junction of the outer and inner rod-segments. The flask-shaped 

 cones are devoid of visual purple; the outer segment exhibits also longitudinal 

 striation, and breaks up readily into transverse discs. In the macula lutea (the 

 yellow pigment of which lies only in the outer retinal layers, and not in the cones) 

 cones alone are present; near the macula each cone is surrounded by a garland 

 of rods. The greater the distance from the macula, the fewer are the cones. 

 Nocturnal animals (owl and bat) possess either no cones whatever, or only im- 

 perfect forms. In birds the retina has many cones, in the lizard cones alone. The 

 rods and cones rest on the sieve-like, fenestrated external limiting membrane 

 (Le) ; both send processes through the openings: the cones to the larger cone- 

 granules, and those lying at a higher level, the rods to the transversely striated 

 rod-granules. The granules belong to : (2) The outer nuclear layer (du K~) ; this 

 and all the succeeding layers, are designated the cerebral layers. There then 

 follows: (3) The narrow outer reticular (granular, plexiform) layer. (4) The 

 inner nuclear layer (inK). The nuclei represent bipolar ganglion-cells (ganglion 

 of the retina) and are called rod-bipolars or cone-bipolar s, whose course is shown 

 in Fig. 269, E. Each bipolar sends out, in addition, a fine fiber between the vis- 

 ual cells, and ends with a punctate knob near the limiting membrane. Ganglion- 

 cells without demonstrable neurites, called amacrine cells, are of unknown nature. 

 (5) The inner reticular (granular, plexiform) layer (in.gr). (6) Ganglion-cell layer 

 (ganglion of the optic nerve) (Ggl). Finally: (7) The layer of optic-nerve fibers 

 (o) , which is next to the internal limiting membrane (Li) . According to Salzer 

 there are in all 438,000; according to W. Krause, however, 400,000 broad and an 

 equal number of the finest optic-nerve fibers. For each fiber there are 7 or 8 

 cones, about 100 rods, and 7 pigmented cells (of the choroid). The fibers are 

 naked axis-cylinders; they are absent in the macula lutea, where, however, the 

 ganglion-cells are numerous. 



The newer investigations have shown that there is no uninterrupted fiber- 

 connection between the rods and cones and the optic-nerve fibers. According 

 to Ram6n y Cajal (Fig. 269) the fibers arising from the rods (a) end in the outer 

 reticular layer (C) as tiny knobs, after passing through the outer nuclear layer(d) : 

 the cone-fibers below the cone-granules (c) like unraveled threads (z) . The bi- 

 polar processes of the internal nuclear layer (e E) break up into fibrils in the outer 

 reticular layer (C) and in the inner reticular layer (F) , which are only approxi- 

 mately in contact, on the one hand with ganglion-cell processes (r), on the other 

 hand, with the elements in the outer reticular layer (C). From each ganglion- 

 cell (i, k) an axis-cylinder process is sent off centripetally, while one or more 

 dendrites enter the inner reticular layer. The optic-nerve fibers contain also 

 a number of centrifugal fibers (s, s). The further course of the optic nerve is 



