

740 STRUCTURE OF THE LENS. 



the skate has no cones, but only rods. The same is the case in the shark and sturgeon, 

 hedgehog, bat, and mole.] , 



[Distribution and Regeneration of Rhodopsin. Keep a rabbit in the dark for some time, 

 kill it remove its eyeball, and examine its retina by the aid of monochromatic (sodium) light. 

 Tin- retina will be purple-red in colour, all except the macula lutea and a small part at the ora 

 serrata The pigment is confined to the older segments of the rods. It is absent in pigeons, 

 hens and one bat, although the last has only rods. It is found both in nocturnal and diurnal 

 animals. Its colour is quickly bleached by light, and it fades rapidly at a temperature of 50 

 to 76 C while trypsin, alum, and ammonia do not affect it. It is restored in the retina by 

 the action of the retinal epithelium. If the retinal epithelium or choroid be lifted off from an 

 excised eye exposed to light, the purple is destroyed ; but if the eye be placed in darkness and 

 tiie retinal epithelium replaced, the colour is restored.] 



Chemistry of the retina. The reaction of the retina, when quite fresh, is acid, and becomes 

 alkaline in darkness. The rods and cones contain albumin, neuro-keratin, nuclein, and in the 

 cones are the pigmented oil globules, the so-called " chromophanes. " The other layers contain 

 th- constituents of the grey matter of the brain. ; 



[Cones. There is no colouring matter in the outer segment of the cones, but in fishes, rep- 

 tiles, and birds the inner segment contains a globular-coloured body often red and yellow, the 

 pigment being held in solution by a fatty body. Kiihne has separated a green (chlorophane), 

 a yellow (xanthophane), and a red (rhodophane) pigment. They all give a blue with iodine, 

 and are bleached by light (Schwalbe).] 



The crystalline lena is enclosed in a transparent capsule, thicker anteriorly than posteriorly, 

 and it is covered on the inner surface of the anterior wall by a layer of low epithelium. Towards 

 the margin of the lens, these cells elongate into nucleated fibres, which all bend round the 



margin of the lens, and on both sides of the lens abut with 

 their ends against each of the triradiate figures. The lens 

 fibres contain globulin enclosed in a kind of membrane. 

 Owing to mutual pressure, they are hexagonal when seen in 

 transverse section (fig. 522, 2), while in many animals, especi- 

 ally fishes, their margins are serrated [the teeth dovetail 

 into each other]. For the sake of simplicity, we may regard 

 the lens as a biconvex body with spherical surfaces, the 

 posterior surface being more curved. As a matter of fact, 

 the anterior part is part of an ellipsoid formed by rotation 

 on its short axis. The posterior surface resembles the section 

 of a paraboloid, i.e., we might regard it as formed by the 

 rotation of a parabola on its axis (Briicke). The outer layers 

 of the lens have less refractive power than the more internal 

 layers. The central part of the lens or nucleus is, at the 

 same time, firmer, and more convex than the entire lens. 

 The margin of the lens is always separated from the ciliary 

 processes by an intermediate space. 



[Chemistry. The lens contains about two- thirds of its 

 weight of water, while its chief solid is a globii lfch called by 

 BerzeTTus crystallin (24 '6 per cent.), Avith a little serum- 

 albumin, salts, cholesterin, and fats.] 



[Cataract. Sometimes the lens becomes more or less 

 opaque, the opacity beginning either in the middle or outer 

 parts of the lens. This is generally due to fatty degenera- 

 tion of the fibres, cholesterin being deposited. An opaque, cataractous condition of the lens 

 may be produced in frogs, by injecting a solution of some salts or sugar into the lymph-sacs ; 

 the result is that these salts absorb the water from the lens, and thus make it opaque. The 

 cataract of diabetes is probably produced from the presence of grape-sugar in the blood.] 



The zonule of Zinn, at the ora serrata, is applied as a folded membrane to the ciliary part of 

 the uvea, so that the ciliary processes are pressed into its folds, and are united to it. It passes 

 to the margins of the lens, where it is inserted by a series of folds into the anterior part of the 

 capsule of the lens. Behind the zonule of Zinn, and reaching as far as the vitreous humour, is 

 the canal of Petit. The zonule is a fibrous perforated membrane. According to Merkel, the 

 canal of Petit is enclosed by very fine fibres, so that it is really not a canal but a complex com- 

 municating system of spaces (Oerlach). Nevertheless, the zonule represents a stretched mem- 

 brane, holding the' lens in position, and may therefore be regarded as the suspensory ligament 

 of the lens. 



Opacity or cloudiness of the lens (grey cataract) hinders the passage of light into the eye. 

 Aphakia, or the absence of the lens (as after operations for cataract), may be remedied by a 

 pair of strong convex spectacles. Of course, such an eye does not possess the power of accom- 

 modation. 

 The vitreous humour, as far as the ora serrata, is bounded by the internal limiting membrane 



Fig. 522. 



1, Fibres of the lens; 2, transvers 



sections of the lens fibres. 



