077 



EYE. 



EYE. 



678 



its function with reference to the surface of the aqueous humour 

 When the lens is hardened in spirit or boiling water, this capsule 

 retains its nature ; and if peeled off, shrivels up and curls upon itself 

 It may be easily detached with a pin from the hard spherical lens o 

 a boiled fish, and will be found still possessed of its peculiar qualitief 

 of transparency and elasticity. The capsule is firmly attached to tht 

 hyaloid membrane behind, from which it is not easy to separate i 

 neatly. Whether it has any further connection than that of mere 

 contact with its own contents, is not easy to show, or to doubt; bu 

 the nature of that connection, if any exists, might be ex pected to be 

 obscure, considering that both the parts are diaphanous, and one o: 

 them almost liquid at the surface of contact. Indeed it has been 

 supposed that a small quantity of limpid fluid was actually interposec 

 V)_tween the lens and its capsule, but this is now shown to be the 

 result of imbibition after death. 



IV. S. 



Magnified view of the Crystalline Lens, showing its laminated and fibrous 

 tiucture, and the direction of its planes of cleavage. 



The capsule of the lens when examined under the microscope is 

 found to consist of two elements, the proper capsule and epithelium. 

 The former is a perfectly structureless and transparent highly elastic 

 membrane, inclosing the lens on all sides as if moulded to it, and 

 parting it from the neighbouring structures. The epithelium is placed 

 on the inner surface of the capsule towards the lens, lining the ante- 

 rior half of the capsule, with a single layer of beautifully clear 

 polygonal cells, of 0-006'" O'Ol"', with round nuclei. The lens itself 

 consists entirely of elongated flat hexahedral elements, 0'0025'" 0'005'" 

 broad and 0-009"' 0-0014'" thick, of a perfectly transparent aspect, 

 very flexible and soft, and having a considerable degree of toughness, 

 which have usually been described as the fibres of the lens, although 

 they are nothing more than thin-walled tubes with clear viscous albu- 

 minous contents, which when the tubes are torn escape from them in 

 the form of large irregular drops, and consequently might suitably 

 be described as the tubes of the lens. These bodies become opaque 

 by the application of all agents that coagulate albumen, as nitric acid, 

 alcohof, heat, chromic acid, and creosote. In caustic alkalies and acetic 

 acid they are quickly dissolved. The concentric lamina;, which con- 

 sist of these tubes, are not regularly defined layers, and never 

 consist of a single stratum of tubes. The course of the tubes in the 

 separate lamellae is in general such that both the superficial and the 

 deeper in the centre of the lens radiate towards the margins, and 

 then curve round upon the other surface anterior or posterior, but in 

 such a way that no fibre extends through the entire seraicircumference 

 of the lens, or reaches, for instance, from the middle of the anterior 

 surface to that of the posterior. More precisely described, the tubes 

 on the anterior and posterior surfaces of the lens do not proceed 

 exactly to the middle, but terminate in a stelliform fissure (fiy. 8) 

 which exists in that situation. 



Aqueous Humour. This fluid, in DO respect distinguishable from 

 water except in holding a minute proportion of several saline ingre- 

 dients hi solution, occupies the space between the lens and the cornea. 

 The iris divides this space into two unequal portions called the 

 anterior and posterior chambers of the eye, and so closely approaches 

 the lens that near the margin of the pupil the two surfaces are sepa- 

 rated by a mere film of aqueous humour. The cavity is lined through- 

 out by a serous membrane, which secretes and limits the fluid, and 

 prevents it from acting injuriously upon subjacent parts of importance. 

 At least a membrane of this kind may be peeled off in some animals ; 

 its existence in the human eye is rather a matter of inference than 

 proof. 



Iris. (Fi'ji. 5, A ; 9, 6.) In speaking of the choroid we have 

 already adverted to the flattened ring called the ciliary ligament or 

 muscle (fig. 5, i), which connects it in front with the sclerotic. The 

 iris arises from the anterior margin of this ring, and is extended, as 

 we hare seen, across the aqueous humour in the form of a thin 

 partition with a round aperture, or pupil, of variable size in the 

 centre, or a little nearer the inner side, the function of which, we 

 need hardly repeat, is to regulate the quantity of light admitted 

 into the eye, by contracting when it is in excess, and dilating when it 

 falls short of the due amount. 



The external appearance of the iris is too familiar to need a 

 particular description. It is covered in front with a glistening 

 polished membrane. The brilliancy of the eye depends in a great 

 measure upon the light reflected by this surface, and is lost when its 



smoothness and transparency are impaired by inflammation. The 

 posterior surface of the iris is called the uvea. (Fir/. 9, 4.) It is 

 thickly coated with pigment, which is prevented from diffusing itself 

 in the aqueous humour by a membrane like that on the choroid. 

 Such a provision is particularly needed here on account of the quick 

 movements of the part in a watery fluid. The colouring matter of 

 the iris has much analogy with the pigment. Like that substance it 

 forms no part of the texture it pervades ; and when the outer mem- 

 branes are removed by maceration in water it may be washed away. 

 Both have a relation in quantity as well as in depth of tint to the 

 complexion and colour of the hair. In the negro the iris is of so 

 dark a hue that it can scarcely be distinguished from the pupil ; while 

 in the white rabbit and other albinoes, including the human variety, 

 where the pigment is entirely wanting from some original malforma- 

 tion, the substance of the iris is transparent, and reflects only the 

 pink colour of the circulating blood. Such eyes are dazzled by a 

 strong light, and probably see better than others in the dusk. The 

 iris, if minutely injected, appears, like the cnoroid, to be composed 

 almost entirely of vessels. It is principally supplied by the two long 

 ciliary arteries (fig. 6) which pierce the sclerotic about half an inch 

 from the optic nerve on either side ; and passing between that mem- 

 brane and the choroid divide near the edge and in the substance of 

 the ciliary muscle, and are wholly distributed to the iris. Their 

 branches are disposed in two conspicuous circles on the front surface, 

 one near the outer or ciliary margin, the other not far from the pupil. 

 But though the iris resembles the choroid in vascularity, it differs 

 essentially from it in other respects. It is richly supplied with 

 nerves, which proceed to the iris and are distributed upon it much iu 

 the same way as the arteries, and are the medium of its sympathy 

 with the retina, and the source of its irritability. It also possesses a 

 peculiar contractile power, which is dependent on smooth muscular 

 fibres of precisely the same kind as those found in the ciliary muscle 

 of the choroid. These fibres are disposed circularly in front and at 

 the fore edge, and in a radiated form behind. (Fig. 9, 6.) (See 

 Lister on the Contractile Tissue of the Iris, ' Quarterly Microscopical 

 Journal,' vol. i.) 



Fig. 9. 



Magnified view of a vertical section of the globe, showing the ciliary body 

 and processes with the uvca, as seen from behind when the lens is removed. 



a, pupil j 6, uvea, or back part of the iris ; c, processes of the ciliary body ; 

 I, ora serrata of the ciliary body, to which a few shreds of the vascular web of 

 he retina remain attached. 



Pupil. (Fig. 9, a.) The pupil in the human eye is bounded by a 



iharp well-defined circular edge. In other animals its shape is subject 



o many varieties which may often be explained by a reference to 



..heir habits and circumstances. In fish it is generally cresceutic or 



mperfectly quadrangular. In herbivorous animals, which often 



continue to browse during the night, it is oblong and obliquely 



ransverse, as in the horse and sheep. In most serpents and many 



rapacious quadrupeds, both aquatic and terrestrial, the pupil though 



arge and round at night is a mere vertical slit when seen by day, 



especially in the smaller species of each genus, as in the common cat. 



t is curious that in the larger cats, as the lion and tiger, as well as in 



ome of the larger four-footed reptiles, the pupil again becomes 



ircular. In all birds, we believe, the pupil is round ; and it may be 



ibserved that, with few exceptions, they all sleep after night-fall. In 



he few nocturnal species, as the owls, the pupil is very large though 



till round, and these birds always shun the day. The long narrow 



mpil is in fact a provision for a greater variation in size than the 



ilrcular form permits, and is generally found in those animals which 



oam at night and also see well by day. When absent iu such 



animals the bulk of the organ is commonly sufficient to secure the 



admission of a sufficient quantity of light after sunset without this 



rovision. In the fcetus the pupil is closed by a vascular film called 



he membrana pupillaris, one function of which is precisely that of 



he centering of a bridge to support and extend it during the process 



>f its construction. A tubular film of the same kind has been 



ately discovered by Mu'ller stretched between the margin of the 



upil and the ciliary body. Both these films are absorbed before birth. 



