50 THE EYE. 



kinds of rods, one kind having a long outer segment of the usual red colour (when not exposed 

 to light), whilst in the others the inner segment is lengthened and either fine or of the usual 

 thickness, whilst the outer segment is short and of a green colour. The rod-fibre is straight 

 in the red variety and in those of the green kind with the larger inner segments, but oblique 

 in the green rods, with finer inner segments. 



In the frog the cones were observed by Engelmann to shorten on exposure to light and to 

 lengthen in the dark (figs. 55. 56). This change occurs through the nervous system, for it 

 will take place equally well if the head of the animal be kept in the dark and only the skin 

 of the trunk and limbs exposed to the action of light. 



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

 and was formerly described with the choroid coat, consists of a single stratum of 

 hexagonal epithelium cells separated from one another by a perceptible amount of 

 clear intercellular substance (fig. 57). The outer surface of each cell that which 

 is turned towards the choroid is smooth and flattened, or slightly convex, and the 

 part of the cell near this surface is devoid of pigment, and contains the nucleus 

 (figs. 55, 56) ; the inner boundary, on the other hand, is not well marked, for the 

 substance of the cell, which here is loaded with pigment, is prolonged into fine, 

 straight, filamentous processes (fig. 57, &), which extend for a certain distance 

 between and amongst the outer segments of the rods and cones indeed the 

 outer parts of the rods may be said to be altogether imbedded in the pigment- 

 cells (c). 



The pigment-cells are not everywhere quite regularly hexagonal, but here and 

 there cells are found, singly or in patches, which are larger or smaller than the rest, 

 and of a more rounded or of an irregularly angular shape. 



The pigment granules, which are in the form of minute crystals, 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 distribution of the pigment granules 

 within the cells varies during life and immediately after removal of the eye 

 according as the retina has been shaded from the light or exposed to its influence. 

 In the former case the pigment is mainly accumulated in the body of the cell (or 

 at least its inner zone), and is withdrawn to a great extent from between the 

 rods ; but after exposure to light, a large amount of pigment is found between the 

 rods, and some of the granules may even extend as far as the external limiting 

 membrane (fig. 56). This has the effect of causing the pigmentary layer to adhere 

 more firmly to the rest of the retina than when the pigment granules are accumu- 

 lated in the body of the pigment-cell. The pigment appears to have inter dliis 

 the function of renewing the colour (visual purple) of the outer segments of the 

 rods after these have become bleached from exposure to the light. This renewal 

 of the colour will take place for a short time after the death of the animal, or the 

 excision of the eye (Kiihue). 



In some animals, e.g., frog, coloured oil-droplets and particles of a highly 

 refracting myelin-like substance (myeloid granules, Kiihne) occur in the non- 

 pigmented portion of the cells, which are further covered next to the choroid by a 

 clear homogeneous cuticular layer. 



The intervals between the rods and cones are only partially filled by the pro- 

 cesses of the hexagonal pigment-cells ; the remaining part appears to be occupied 

 by a clear substance, which, according to Henle and H. Miiller, is of a soft elastic 

 consistence during life and in the fresh condition, but soon liquefies after death ; 

 but according to Schwalbe, is normally liquid. In the embryo, between the 

 hexagonal pigment and the remainder of the retina, there is a distinct cleft filled 

 with fluid (remains of cavity of primary optic vesicle), homologous with the 

 ventricular cavities of the brain, with which it is originally in continuity. 



The sustentacular tissue of the retina : Miilleriaii or radial fibres. 



