EYE. 



181 



black, and so loosely connected with the struc- 

 ture in whicli it is de|K>sited, that in dissecting 

 the eyes of our common graminivorous animals 

 underwater it becomes diffused, and colours the 

 fluid as the ink of the cuttle-fish obscures the 

 water into which it is shed. It is not confined 

 to any one particular structure, but is deposited 

 in every situation where it is necessary for the 

 purpose for which it is destined. It is found 

 in considerable quantity on the inner surface of 

 the choroid, where it appears as if laid on in 

 the form of a paint, arid is frequently so 

 described ; but it is much more probable that 

 it is deposited in the interstices of the exqui- 

 sitely fine cellular membrane which connects 

 the choroid with the delicate covering of the 

 retina. In this situation it often, especially in 

 infants, presents the appearance of a perfectly 

 distinct black membrane, which may be peeled 

 off in flakes or allowed to remain on the retina 

 in patches, as noticed by Ilaller. It also per- 

 vades the structure of the choroid, at least in 

 the adult, and even stains the inner surface of 

 the sclerotic and the cellular layer which con- 

 nects these two membranes. It is deposited 

 in larger quantity in the ciliary processes and 

 upon the back and in the texture of the iris. 

 In many animals it is found forming a black 

 ring round the margin of the cornea and in the 

 edge of the third eye-lid, as well as in the 

 pecten or marsupium nigrum in birds. It is 

 even sometimes found scattered, as if acci- 

 dentally, as in the texture of the sclerotic in 

 hogs, and within the sheaths of the optic nerve 

 in oxen ; it is obvious that it does not require 

 any special form of organization for its produc- 

 tion, but is merely secreted into the cellular 

 membrane, where necessary, as the colouring 

 matter is secreted with cuticle on the skin. 



It is darker in the earlier periods of life, and 

 in the infant is more confined to the inner sur- 

 face of the choroid and to the posterior surface 

 of the iris, than pervading the texture of either 

 of these membranes. In old age it evidently 

 fades, and even appears as if absorbed in 

 patches. It is sometimes altogether absent, as 

 in those animals called albinos, where all the 

 parts usually coloured are unstained. Its use 

 is obviously to prevent the rays of light from 

 In'ing reflected from surfaces where they should 

 ! absorbed, a provision as essential to the 

 perfection of the animal eye as to the artificial 

 optical instrument. It is also applied to give 

 complete opacity to prevent the transmission of 

 liylit, and hence is deposited in large quantity 

 in and on the iris, as well as in the ciliary pro- 

 cesses which correspond in situation to the 

 excised part of the sclerotic, through which 

 the light might otherwise pass to the bottom of 

 the eye, and disturb correct vision. The layer 

 of buck pigment on the inner surface of the 

 choroiil has undergone a careful microscopic 

 investigation, especially by Mr. T. W. Jones, 

 the results of which are stated in a short 

 account of the anatomy of the eye prefixed to 

 the second edition of Mr. M'Kenzie's work on 

 Diseases of the Eye. He says that it possesses 

 Organization and constitutes a real membrane, 

 and when examined with the miscrostope " is 



seen to consist of very minute flat bodies of a 

 hexagonal form, joined together at their edge*. 

 These bodies, which are about ^th of an inch 

 in diameter, consist of a central transparent 

 nucleus, surrounded by an envelope of colour- 

 ing matter, which is most accumulated at their 

 edges. The centre, indeed, of each hexa- 

 gonal plate is a transparent point, and appears 

 somewhat elevated, the elevations on the inner 

 surface coi 'responding to depressions to be 

 described in the membrane of Jacob. That 

 part of the membrane of the pigment situated 

 on the pars mm plicata of the ciliary body 

 around the ciliary processes, and on the poste- 

 rior surface of the iris, is composed of irregu- 

 larly rounded bodies, analogous to the hexa- 

 gonal plates. In albinos the same membrane 

 exists, but contains no pigment. The bodies 

 composing it are but little deve- 

 Fig. 1 06. loped, being nothing but the central 

 nuclei separated from each other by 

 large intervals, and not hexagonal, 

 but circular, or even globular.' 1 The 

 annexed figure represents this mem- 

 brane of the pigment as described. 

 Sometimes the black pigment is totally or 

 partially deficient, not only in inferior animals, 

 out also in man, constituting the variety deno- 

 minated albino, of which the white rabbit 

 affords a good example. The circumstance has 

 attracted considerable attention, and has been 

 the subject of particular observation by Mr. 

 Hunter, Blumenbach, and many others. Dr. 

 Sachs has given a curiously elaborate account 

 of himself and his sister, who are both albinos. 

 The eye in such cases appears of a beauti- 

 fully brilliant red, in consequence of the blood 

 being seen circulating through the transparent 

 textures unobscured by the pigment, but the 

 individual suffers from the defect in conse- 

 quence of the light being transmitted through 

 all the exposed part of the organ; proving 

 that the covering of black pigment is deposited 

 on the back of the iris and in the ciliary pro- 

 cesses to obviate this injurious consequence. 

 In human albinos the eyes have often a tremu- 

 lous oscillating motion, and the individual is 

 unable to bear strong light. 



The colour of the black pigment does not ap- 

 pear to depend on the presence of carbon or 

 other dark material, and the minute quantity 

 of oxide of iron contained in it is obviously 

 insufficient for the production of so deep a tint. 

 It is insoluble in water, either hot or cold, or 

 in dilute sulphuric acid ; but strong nitric or 

 sulphuric acids decompose it, and are decom- 

 posed by it. Caustic potash is said to dissolve 

 it, though with difficulty, but as ammonia is 

 evolved during the process, and the nature of 

 the pigment necessarily altered, it cannot be 

 considered a case of simple solution. By 

 destructive distillation it affords an empyreu- 

 rnatic oil, inflammable cases, and carbonate of 

 ammonia. It is, therefore, obviously an ani- 

 mal principle ./ /H'lifrii, its elements being 

 oxygen, hydrogen, carbon, and nitrogen. One 

 hundred parts in a dry state leave, when 

 incinerated, 4.46 of a calx, consisting of chlo- 

 ride of calcium, carbonate of lime, (diusphaUi 



