Sometimes the black pigment is totally or partially deficient, not only in inferior animals, but 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. Bunter, Blumenbach, and many others. Dr. Sachs has given a curiously elaborate account of himself and his sister, whn 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 matic oil, inflammable gases, and carbonate of ammonia. It is, therefore, obviously an ani mal principle sui generis, its elements being oxygen, hydrogen, carbon, and nitrogen. One hundred parts in a dry state leave, when incinerated, 4.46 of a mix, consisting of chlo ride of calcium, carbonate of lime, phosphate of lime, and peroxide of iron. For these par ticulars I am indebted to Dr. Apjohn.
Of the iris.—This is the circular partition or screen interposed between the cornea and crys talline lens, filling up the aperture in the ante rior part of the sphere of the choroid, and conse quently exactly fitted to the place of union of the ciliary ligament and choroid with the sclerotic round the cornea. It has an aperture in the centre called the pupil, through which the central portion of the pencil of rays incident upon the cornea is transmitted, while the extreme rays are intercepted ; and appears to answer the same purpose as the diaphragm or eye-stop in the telescope, but with this advantage, that it is enlarged or diminished according to the quantity of light, the distance of objects, or even the will of the individual. The iris is frequently called uvea, a term also applied to the spherical choroid ; or the anterior part is called iris, and the posterior uvea. To avoid confusion the term should be discarded alto gether, and that of iris alone retained to designate this important part of the organ.
The surface of the iris is flat or plane, al though it appears convex when seen through the cornea, or when in dissecting the eye it falls on the convex surface of the crystalline lens. It is remarkable that the aperture or pupil is not exactly in the centre of the disc, but a little towards the inside. The anterior surface presents a very peculiar and remarkable appearance, evidently not depending on or arising from vascular ramifications or nervous distribution. This appearance is described with precision and accuracy both by Zinn and Ilaller, although unnoticed or only briefly al luded to in many of the slovenly compilations which have appeared since they wrote. It is,
however, described by Meckel, who saw what he describes, and read what he quotes. Haller's words are as follow :—" In anteriori lamina iridis eminet natura flocculenta, vane in flam mulas quasdam introrsum euntes disposita, quibus aliqua est similitudo rotundorum ar euum, ad centrum pupillw convexorum. Qui vis flocculus est serpentinarum striarum intror sum convergentium, et intermistarum macu larum fuscarum congeries : conjuncti vero flocculenti fasciculi arcum quasi serratum, emi nentem, ad aliquam a pupilla distantiam effi ciunt, qui convexus eminet, quasi antrorsum, supra reliquum planum popi1finelatus. Fabrics: pulchritudinem nulla icon expressit." (Ele menta Physiologim, torn. v. p. 369.) Zinn's description is equally accurate and precise. In the 12th volume of the Medico-Chirurgical Transactions I have noticed this structure in the following words: " If the iris be attentively examined in the living subject, or under water after the cornea has been removed, a number of irregularly shaped masses may be seen pro jecting from the middle space between the circumference and the pupil. From the con vexities of these masses, a number of elevated lines, equally irregular in size and number, proceed toward the pupil, and attach them selves at the distance of about a part of an inch from its margin, and from this point of attachment a number of much smaller stria> converge to the edge of the central open ing. It is quite impossible for words to give an adequate idea of this appearance. If I ventured to compare it with any other with which I am acquainted, I should say that it resembled strongly the earnee cabin:7z˘ and corda tendinee of the heart, both in form, arrangement, and irregularity of conformation. This structure is more strongly marked in the hazel than in the blue iris ; and in many cases the fleshy projections coalesce, by which they appear less distinct ; but the loops or cords which arise from them always exist, and often project so much from the plane of the iris as to admit of having a small probe or bristle passed beneath them. That this appearance of the iris does not depend on any particular disposition of its vessels, is, I think, obvious, from the thickness of these cords or striae being so much greater than the vessels of the iris, from their being arranged in a manner altogether different from vascular inosculation, and finally, because the iris when successfully injected and expanded does not present that interlacement of branches surrounding the pupil which has so often been described from observation of its uninfected state." The anterior surface of the iris is of a light blue colour in persons of fair skin and light hair, of a blue grey in others, sometimes of a mixture of tints called a hazel iris ; and in negroes and others, where the skin is stained by the usual colouring matter, the iris is of a deep brown, and is commonly described as a black eye, being pervaded by the black pigment throughout its texture, as well as coated with it on its posterior surface. In animals altogether destitute of the usual colouring matter on the surface, called albinos, the iris has no other colour than that of the blood which circulates in its vessels. The annexed engraving is a copy of a most accu rately executed representation of the face of the iris, shewing the earned' columnar and corde tendinee much magnified.