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 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 b 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." IIaller 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 pcds,it is obvious that they arc 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. The 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 Ituysch, 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, hut 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.
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 hailer operates to the pre sent day to throw a doubt upon the muscula rity of the iris ; but hailer, strange as it may appear, was not correctly informed in many particulars respecting this structure. Ile de nies the existence of the orbicular muscle ; he doubts the irritability of the organ, and he even considers it destitute of sensibility, and as sumes that the pupil is dilated after death. Any anatomist may, however, demonstrate the orbicular muscle; any surgeon breaking up a cataract, may elicit the irritability, and see the pupil contract, as the fragments of the lens or the side of the needle touch its margin. The pain produced by pinching or cutting the iris in operations for cataract and artificial pupil is no longer matter of doubt, and the assumption that the pupil dilates when death takes place is disproved by daily observation. The pupil
contracts to exclude light when too abundant, and dilates to admit it when deficient in quan tity ; the heart contracts to expel the blood, and dilates to receive it; the diaphragm con tracts to fill the lungs, and relaxes to assist in emptying them. I can see no material differ ence between the phenomena exhibited by the actions of the iris, and those displayed by the muscular system generally. I believe that when the pupil contracts to intercept light, that con traction is accomplished by the orbicular mus cle, which operates as any other sphincter ; and that when the pupil is dilated to admit light, the dilatation is accomplished by the con traction of the structure, which I have said re sembles the earnec columnee and cords' tendines in the heart.
During foetal life the aperture in the centre is closed by a membrane, hence technically called membrane pupillaris. The discovery of this membrane was first announced by Wachendorf, but was subsequently claimed by Albinus, and still later by Dr. Bunter for a person of the name of Sandys. It is usually described as existing from the earliest period of foetal life to the seventh month, when it disappears. In the paper communicated by me to the Medico Chirurgical Society, I have endeavoured to shew that this description is not correct, but that this membrane continues to the ninth month. The account there given is as follows : " If the eye be examined about the fifth month, the membrane pupillaris is found in great perfec tion, extended across a very large pupil ; the vessels presenting that singular looped arrange ment, (with a small irregular transparent por tion in the centre,) well depicted by Wrisberg, Blumenbach, Albinus, Siimmerring, Cloquet, and others. About the sixth mouth it is equally perfect; the pupil is however smaller, the iris being more developed. Subsequently to this (late the vessels begin to diminish in size and number, and a larger transparent portion occu pies the centre. At the approach of the eighth month, a few vessels cross the pupil, or ramify through the membrane at a short distance from the margin, without at all presenting the looped appearance of the previous period, but ad mitting a free communication between the ves sels of the opposite side of the iris. The pupil is now still more diminished in size, and the iris has assumed its characteristic coloured ap pearance; notwithstanding the absence of ves sels, the membrane still preserves its integrity, though perfectly transparent. The period now approaches when it is to disappear; this occur yence takes place, according to my observations, a short time previous or subsequent to birth. In every instance where I have made the exa mination, I have found the membrane papilleris existing in a greater or less degree of perfection in the new-born. infant ; frequently perfect without the smallest breach, sometimes pre senting ragged apertures in several places, and, in other instances, nothing existing but a rem nant hanging across the pupil like a cobweb. I have even succeeded in injecting a single ves sel in the membrane pupillaris of the ninth month. Where I have examined it in subjects who have lived for a week or fortnight after birth, as proved by the umbilicus being healed, I have uniformly found a few shreds still re maining. It is obvious from the preceding observations, that the membrane does not dis appear by a rent taking place in the centre, and retraction of the vessels to the iris, as sup posed by Blumenbach, but that it at first loses its vascularity, then becomes exceedingly thin and delicate, and is finally absorbed. The de monstration of what I have advanced respect ing this delicate part is attended with much difficulty, and requires great patience. The display of the membrane pupillaris of the seventh month is comparatively easy ; hut at the ninth month, or subsequently, it can only be accom plished by particular management. The eye, together with the appendages, should be care fully removed from the head ; it should then be freed from all extraneous parts by the scis sors, under water, and a careful section made at a short distance behind the cornea ; taking care to include the vitreous humour in the divi sion, in order that the lens may remain in its proper situation. The portion to be examined should now be removed into a shallow vessel of water, to the bottom of which a piece of wax has been secured. The operator should be provided with fine dissecting forceps and nee dles in light handles ; with one needle he should pin the sclerotic down to the wax, and with the other raise the lens, and portion of vitreous humour attached to it, from the ciliary processes, and separate the ciliary ligament from the sclerotic. He may now expect to dis cover the inembrana pupillaris, but its perfect transparency renders it completely invisible; he may, however, ascertain the existence, by taking a minute particle of the retina and dropping it into the centre of the pupil, where it remains suspended if this membrane exist. The preparation should now be taken up in a watch-glass, and placed in a weak mix ture of spirit and water, and a little pow dered alum raised on the point of a needle dropped upon it. After a day or two it may be examined ; and if the membrane be pre sent, it has become sufficiently opaque to be visible, and may now be suspended in a bottle of very dilute spirit." In the annexed engravings, A represents the inembrana pu pillaris of about the fifth month, present ing the peculiar looped arrangement of the vessels. B represents the membrane about the eighth month, not presenting the looped ar rangement. C represents the membrane with a red vessel in its structure at the ninth month. D The pupil is closed by this membrane during focal life in order to preserve its dimensions, and secure a correct growth of the iris while the organ is in darkness. If the membrane disap peared about the seventh month, the pupil should become dilated and remain so during the two succeeding months, unless the muscu lar power be undeveloped, which is not proba ble, as it may he seen to operate shortly after birth.