182 



EYE. 



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 

 Haller, 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. Mailer'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- 

 cuum, ad centrum pupillse convexorum. Qui- 

 vis flocculus est serpentinarum striarum intror- 

 sum convergentium, et intermistarum macu- 

 larum fuscarum congeries : conjunct! vero 

 flocculenti fasciculi arcum quasi serratum, emi- 

 nentem, ad aliquam a pupilla distantiam effi- 

 ciunt, qui convexus eminet, quasi antrorsum, 

 supra rehquum planum pupillaeelatus. Fabricae 

 pulchritudinem nulla icon expressit." (Ele- 

 menta Physiologise, 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 twentieth 

 pail 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 earner columns and 

 corda tendinea 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 

 uninjected 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 earner columns and 

 cor da tendinea much magnified. 



Fig. 107. 



The posterior surface of the iris is as remark- 

 able as the anterior, but altogether different in 

 its nature. I have given the following des- 

 cription of it in the paper to which I allude in 



