EYE. 



182 



the Medico-Chirurgical Transactions. " In 

 order to obtain a correct view of the posterior 

 surface of the iris, a transverse vertical section 

 of the eye should be made at the distance of 

 about an eighth of an inch behind the cornea, 

 and the lens, and portion of vitreous humour 

 attached to it, removed : the iris now appears 

 covered by a thick layer of black pigment, 

 marked by a number of converging lines ; 

 these lines on close inspection are found to be 

 channels or hollows, as if resulting from a 

 puckering or folding of the membrane. The 

 pigment is secured from being detached, and 

 diffused in the aqueous humour, by a fine 

 transparent membrane, which is closely attached 

 to the margin of the pupil, from whence it is 

 continued over the back of the iris, and anterior 

 extremities of the ciliary processes, to the cir- 

 cumference of the lens, over the front of the 

 capsule of which it is also probably extended, 

 if it be, as may be supposed, the membrane of 

 the aqueous humour. This delicate membrane 

 may be turned down by the point of a needle ; 

 as it is connected to the iris by loose cellular 

 structure only, in the interstices of which the 

 black pigment is deposited. It is at first black, 

 but by gentle agitation in water the colouring 

 matter is removed, and the membrane remains 

 transparent. When the membrane and pig- 

 ment have been removed, the back of the iris 

 appears free from colour, and marked by a 

 number of delicate elevated folds, converging 

 from the ciliary processes to within a short 

 distance of the pupil ; they are permanent and 

 essential, and seem of the same nature as the 

 ciliary processes. The pupil is immediately 

 surrounded by a well-defined distinct circle, 

 about the twentieth part of an inch in diameter, 

 of a denser structure than the rest of the iris : 

 this is what has been long described as the 

 orbicular muscle, or constrictor of the pupil. 

 If the iris be treated, as I before mentioned, by 

 maceration and extension, this appearance still 

 preserves its integrity, and retains its original 

 character." Haller and Zinn describe these 

 converging radiating folds, but the former de- 

 nies the existence of the circular arrangement 

 round the margin of the pupil, of the presence 

 of which I do not entertain the slightest doubt, 

 but which is sometimes so slightly marked, 

 that I am not surprized to find its existence 

 doubted if the part has not been examined in 

 a variety of examples. This circle, or orbicular 

 muscle, is sometimes equally visible on the 

 anterior surface, but is generally obscured by 

 the converging cords above described. The 

 folds or elevations on the back of the iris, con- 

 verging toward the pupil, have been considered 

 the muscular agents for dilating the pupil, but 

 if examined in the eyes of the larger quadru- 

 peds, it is obvious that they are destined to give 

 this part of the organ the requisite degree of 

 opacity, and to afford an appropriate place for 

 the deposit of the black pigment, in this res- 

 pect closely resembling the ciliary processes, 

 and the pecten in the eye of birds, so much 

 so, that I think they might be appropriately 

 called the ciliary processes of the iris. 



The iris is most plentifully supplied with 



bloodvessels and nerves. Thi two long .ciliary 

 arteries which penetrate the sclerotic posteri- 

 orly advance horizontally, about the middle 

 of the eyeball, between that membrane and the 

 choriod, to the iris, where each divides into two 

 branches, which proceed round the circumfer- 

 ence and inosculate with each other, thus form- 

 ing an arterial circle, from which numberless 

 branches converge to the pupil. Much impor- 

 tance has been attached by anatomists to the 

 manner in which these radiating vessels are 

 disposed, in consequence of the representation 

 of Ruysch, who exhibited them as forming a 

 series of inosculations at a short distance from 

 the pupil, since called the lesser circle of the 

 iris. I do not deny that the vessels of the iris 

 inosculate as in other parts of the body, but I 

 do not believe that they present this very re- 

 markable appearance, and I suspect that 

 Ruysch exaggerated what he had seen, or de- 

 scribed from an iris in which the injection had 

 been extravasated and entangled in the tendi- 

 nous cords, which I have described as extend- 

 ing from the fleshy bodies to the margin of the 

 pupil. The question is fortunately of no 

 importance. It is sufficient to know that the 

 organ is amply supplied with arterial blood. 



The iris is plentifully furnished with 

 nerves : they are derived from the third and 

 fifth pairs, with communications from the sym- 

 pathetic, and consequently having connexions 

 with the sixth. They penetrate the sclerotic 

 posteriorly, and advance towards the iris be- 

 tween the sclerotic and choroid, about fifteen 

 or twenty in number: arrived at the ciliary 

 ligament, they divide at acute angles, and may 

 be traced through this structure until they are 

 finally lost in the iris, as seen in the annexed 

 figure. 



Fig. 108. 



From the foregoing description, it appears 

 that the iris is eminently distinguished for the 

 perfection of its organization ; and endowed as 

 it is with the power of enlarging or diminishing 

 the aperture in its centre, there can be little 

 doubt that it is a beautiful application of mus- 

 cular structure and function to the perfection 

 of this most elaborately constructed organ. 

 The authority of Haller operates to the pre- 

 sent day to throw a doubt upon the muscula- 

 rity of the iris ; but Haller, strange as it may 

 appear, was not correctly informed in many 

 particulars respecting this structure. He de- 

 nies the existence of the orbicular muscle ; he 

 doubts the irritability of the organ, and he even 



