The cells are flattened, six-sided, and form a pavement covering the outer portions of the rods and cones, and sending down long processes between them.

Externally the cells consist of a layer of neurokeratin; internally they are protoplasmic; in the protoplasm are found one or two nuclei

FIG. 73.-Pigmented Epithelium of the Human Retina (Max Schultze) highly magnified. a, Cells seen from the outer surface. b, Two cells in profile with fine offsets extending inwards. c, A cell still in connection with the outer ends of the rods.

and large numbers of black rod-shaped pigment granules. Deposits of a substance called myeloidin by Kühne, and in some animals of yellow fat-globules; are also found.

The black pigment.-Fuscin.-Owing to movements in the cell protoplasm of the nature of amoeboid movements, the granules of black pigment are differently distributed at different times; after keeping a frog for several hours in darkness, the pigment will be found in the cell bodies, and in the parts of the processes nearest to the cell bodies. But if the frog has been exposed for a similar time to sunlight before death, the pigment granules will be distributed chiefly along the processes, and a relatively small number remain in the bodies of the cells themselves. In some animals (dog, cat, &c.) much of the retinal epithelium contains no fuscin, but the cells are filled with fine crystals (Max Schultze); this forms the tapetum. In some fish, e.g. bream, the tapetum (or pseudo-tapetum) contains guanine, a highly refracting substance; while in the ox and sheep the tapetum is merely fibrous tissue (Kühne and Sewall1).

Fuscin is one of the group of black pigments termed melanins. It has been investigated by Berzelius, who found it contained a small quantity of iron, by Scherer, who found no iron, and also by Rosow and Sieber. The percentage composition obtained by the various observers shows great discrepancies, and this, taking also into account their methods of preparing the pigment, renders it probable that they were not dealing with a pure substance. The failure of some observers,

1 Verhandl. der naturhist. Vereins Heidelberg, N.S. ii. Heft v.

2

for instance, to obtain evidence of the presence of iron was due as Mörner points out to their having used hydrochloric acid at one stage or other of their operations; this acid dissolves out nine-tenths of the iron from the pigment. May's method of preparing fuscin is to boil several hundreds of retina in alcohol, then in ether, lastly in water; the residue is subjected to tryptic digestion for twenty-four hours; the pigment, nuclein, and neurokeratin remain undigested; the first-named impurity is dissolved by trituration with alkali, and the last-named must be picked out as well as possible with forceps.

Fuscin dissolves by boiling it a long time with concentrated sulphuric acid, or concentrated caustic alkalis.

Like all the other retinal pigments, fuscin is bleached in the air, only very slowly indeed. This is probably due to oxidation. The physiological relation of the fuscin-bearing cells with the rods and cones will be dealt with in the consideration of those structures.

There is considerable doubt as to whether this pigment is ultimately derived from hæmoglobin; Krukenberg considers it is more closely related to the lipochromes or fatty pigments. It is, however, undoubtedly nitrogenous. It does not belong to the group of brown pigments, many of which occur in plants called humous substances by Hoppe-Seyler, since on fusing with alkali it yields no pyrocatechin or protocatechnic acid (Hirschfeld 4). (See p. 149.)

3

Myeloidin. Myeloidin, or myeloid substance, is not a chemical unit. The term is used as indicating that the cells contain a substance similar to that which forms the white substance of Schwann in nerve-fibres. It is also found in the rods, and will be there more fully dealt with.

Yellow fat-globules.-These are not present in all animals; they are especially abundant in the retina of the frog. The pigment can be extracted by ether, carbon bisulphide, benzene, &c. It shows two absorption bands between F and G. The yellow pigment was called lipochrin by Kühne. It is, however, exceedingly probable that this is the same pigment found generally in adipose tissue; it belongs to the class of pigments called lipochromes or luteins, and like all these pigments is slowly bleached by sunlight.

1 K. A. H. Mörner, Zeit. physiol. Chem. xi. 66–140. In this paper the references to the writings of the observers mentioned above will be found.

2 Untersuchungen aus d. physiol. Inst. der Univ. Heidelberg, ii. 324. 3 Zeit. physiol. Chem. xiii. 66.

4 Ibid. xiii. 407.

The Rods and Cones

The rods and cones form the nerve-epithelium which receives the impressions of light from without. The accompanying figure shows the general shape and relative size of a rod and a cone. Each consists of two distinct segments, an inner and an outer. The outer or narrower segment is doubly refracting, and is stained darkly by osmic

FIG.74.-ARod and a Cone from the

Human Retina (Max Schultze) highly magnified.

acid, while the inner segment is singly refracting, and stains as protoplasm does with carmine, magenta, &c.; the outer part of the inner segment is longitudinally fibrillated, while its inner part is granular or homogeneous; and in the case of the cones has been shown to undergo movements (Engelmann). The prolongation inwards of the rods and cones ultimately join the terminal nerve-fibrils of the optic nerve; the connection occurs in the outer molecular layer (Gunn'). The outer segments of the rods contain the pigment known as the visual purple or rhodopsin; the inner segments of the cones in birds, reptiles, and amphibia contain coloured oil-globules, known as chromophanes.

The following animals have no cones: the ray, shark, sturgeon, bat, hedgehog, and mole. The following have no rods lizards, serpents, tortoises, and perhaps all reptiles. Mammals have more rods than cones, except at the part where vision is most acute, viz. the macula lutea; here cones only are found. Birds have more cones than rods; the owl is an exception to this rule.

The outer segment of a rod can by the action of certain reagents, such as free toluylenediamine or its neutral acetate, be split into a number of cross discs; the indication of such a division can be seen in the fresh rod in the shape of indistinct transverse markings or groovings. Each disc so obtained retains its reddish purple tinge (due to rhodopsin), and is seen to consist of an outer ring of more solid material, filled with a less solid substance. The outer ring is composed of neurokeratin, the inner substance is what stains readily with osmic acid, namely, the myeloidin of Kühne. Myeloidin is not lecithin, as some have supposed; lecithin is not coloured nearly so darkly by osmic acid, nor is the black coloration removed by hydrogen peroxide,

1 Journal of Anat. and Physiol. 1877.

as it is in the case of myeloidin (Unna). Myeloidin is probably a compound or mixture of lecithin and a globulin (vitellin). The myeloidin can be dissolved out by concentrated solutions of certain neutral salts like ammonium chloride (Dreser').

The outer segment of a cone is smaller than that of a rod; the transverse markings are more distinct than in the case of the rods, but the separation into discs does not take place so readily; this is owing, as some have supposed, to the existence of a delicate membrane covering the entire outer segment. The outer segment of the cones contains no visual purple. It consists, however, as that of the rod does, of neurokeratin externally, myeloidin within.

5

Visual purple or rhodopsin. Although H. Muller2 in 1851, and Leidig3 in 1857, noticed the red colour of the retina of the frog, it was not till twenty years later that Boll discovered that the red colour of the living retina disappears under the bleaching influence of light, and that it is restored by darkness, disappearing, however, for good on the death of the animal. The subject was taken up by Kuhne, who found he was able to study the reactions and properties of the substance which give the colour to the retina if observations on it are made in a chamber illuminated only by the sodium flame, yellow light having only a slight bleaching action on the pigment, which he called Sehpurpur (visual purple). It was first found that the pigment was contained only in the outer segments of most of the rods; it was completely absent in the cones, in the rods in the neighbourhood of the macula lutea, and in the rods near the ora serrata, the anterior border of the retina.

A solution of visual purple can be obtained by means of a 2-5 per cent. solution of the bile salts of the ox. The solution so obtained contains also a proteid resembling myosin (Dreser"). Such a solution can be best obtained from frog's retinæ, as it is easy to free these from blood. When evaporated to dryness in vacuo, an amorphous carminelike powder is obtained on which light has very little action; this redissolves readily in a solution of bile salts, and when placed in a dialyser the pigment does not pass through the membrane.

On exposure to light, the visual purple first becomes yellow (visual yellow) and then colourless. The spectroscopic appearance of visual purple and visual yellow is shown in fig. 75, spectra 1 and 2; there

1 Zeit. für Biologie, xxii. 23.

2 Zeit. f. wissensch. Zool. iii. 234.

3 Lehrbuch d. Histologie, p. 238. 4 Monatsber. d. Berlin Akad. 12 Nov. 1876. Kühne, Ewald, Ayres, Mays, and others of Kühne's pupils, published numerous papers on the subject in the Untersuch. aus d. physiol. Inst. zu Heidelberg, vols. i. and ii.

6 Loc. cit.

are no well-defined bands, but a general absorption of the central regions of the spectrum. White light bleaches rhodopsin most quickly, then follows green, blue, and, after an interval, yellow, violet, orange, and red. The sodium flame takes about two hours to bleach a frog's retina, but is more convenient than a red flame, as by light of a red colour it is difficult to detect and avoid blood stains. The intermediate stage of visual yellow is bleached more quickly by rays from the violet end of the spectrum, or it may be that less yellow is produced under the influence of such rays.

The rapidity with which visual purple fades increases with the temperature up to 76° C., at which temperature it disappears instantly even in the dark.

Alcohol, ether and chloroform, caustic alkalis and acids destroy the pigment. Putrefaction and tryptic digestion do not. Oxidising agents, such as ozone, hydrogen peroxide, osmic acid (the black colour produced with myeloidin having first been destroyed by hydrogen peroxide, or the myeloidin may be previously removed by ammonium chloride), ferric chloride, potassium chlorate, and iodate have no effect. These reactions show that visual purple is a substance already highly oxidised.

Such reactions, however, are of little interest compared to those produced by the action of light. That the bleaching action of light occurs during life was most conclusively shown by those experiments in which Kühne succeeded in obtaining what may be compared to photographic impressions upon the retina; these were obtained in rabbits. The animal was first put in darkness by covering its head with a black cloth, it was then exposed to the light of a window, and immediately decapitated, the eyes removed, and the retinal colours fixed by a solution of alum; a small bleached area corresponding in shape to the window, and about a millimetre square, was found on the retina next day. Such optograms may be preserved a long time by drying the retina in vacuo after removal from the alum solution.

Regeneration of visual purple.--This is continually taking place during life, and occurs especially in the dark. This phenomenon appears to be associated with the hexagonal pigment cells which send down their processes between the outer segments of the rods; if a piece of a fresh retina be lifted from the black pigment cells, and then be exposed to the light, it will become bleached, and if then the retina be placed in darkness the colour will not return as it does in the rest of the retina; but if the flap be replaced so as to touch the hexagonal cells, regeneration of the purple occurs. This function of the hexagonal cells does not seem to depend on the amount of fuscin they contain. It is possible