Of the aqueous humour.--in the preliminary observations at the commencement of this article, I stated that a cavity or space filled with water exists between the cornea and crys talline lens, in which space the iris is extended, with its aperture or pupil, to moderate the quantity of light, and Interrupt the passage of the extreme rays. It is bounded anteriorly by the concave inner surface of the cornea, and posteriorly by the crystalline lens and other parts, and is necessarily divided into two spaces or chambers by the iris. That in front of the iris, called the anterior chamber, is bounded by the concave inner surface of the cornea anteriorly, and by the flat surface of the iris posteriorly, which, I have already stated, is a plane, not a convex surface, as represented in the plates of Zinn and others. The size of this space is necessarily small, and varies in different individuals according to the convexity of the cornea, which also frequently varies. It is always, however, sufficiently large to allow the surgeon to introduce a needle to break up a cataract without wounding the iris or cornea. The posterior chamber is bounded in front by the back of the iris, and behind by the crys talline lens; with that portion of the hyaloid membrane of the vitreous humour, which is between the anterior termination of the ciliary processes of the choroid and the circumference of the lens. The circumference of the pos terior chamber is bounded by the anterior ex tremities of the ciliary processes of the choroid, as they extend from the vitreous humour to the back of the iris. It does not appear to be generally admitted or well understood that any part of the hyaloid membrane of the vitreous humour enters into the composition of the posterior chamber of the aqueous hu mour, notwithstanding the decisive opinion and accurate representation of the celebrated Siimmerring, in which I entirely concur, as I have stated above in describing the vitreous humour.
The size of the posterior chamber has been the subject of much discussion and contro versy, and various attempts have been made by freezing the eye and other means to deter mine the matter. Petit, after a careful inves tigation, considered that the distance between the lens and iris was less than a quarter or half a line, in which Haller appears to concur. Winslow, in the Memoirs of the French Aca demy for 1721, insists that the iris is in contact with the lens. Lieutaud, in his Email Ana tomiques, is equally positive on this point, and even denies altogether the existence of a posterior chamber. The question is not an indifferent one, inasmuch as it involves impor tant considerations as to operations for cataract and inflammations of the iris. Modern ana tomists appear, generally, to consider the dis tance between the lens and iris to be greater than it really is. Although I cannot agree with Winslow and Lieutaud that the margin of the pupil is always in contact with the lens, I believe it frequently is so, especially in the earlier periods of life, when the curvatures of the lens are considerable. In iritis adhesions generally take place between the margin of the pupil and the capsule of the lens, a conse quence not easily accounted for, if the parts be not in contact. In old age the lens be
comes much flattened, and therefore retreats from the pupil, to such a degree that the sha dow of the Iris may often be seen in a crescentic form on a cataract ; and in such persons, whe ther from this cause or from the inflammation not being of the adhesive character, blindness is more frequently attended with dilated pupil.
In breaking up cataracts through the cornea, I have repeatedly satisfied myself of the con tact or close vicinity of the two surfaces by placing the needle between them. The an nexed outline section, from the work of merring, skews how small he considered the space between the iris and lens, and displays accurately how the posterior chamber is formed by the iris teriorly, the lens teriorly, and the ary processes at the circumference, with the small circular portion of the hyaloid brane of the vitreous humour between the ciiliary processes of the choroid and the circumference of the lens.
It appears to me unaccountable why sur geons, with these anatomical facts before them, still continue to introduce the needle into the posterior chamber, to break up cataracts, in stead of passing it through the cornea into the anterior chamber, where ample space exists, and a full view is obtained of all the steps of the operation. In doing so the needle is thrust through opaque parts among delicate structures, into a narrow cavity, where, hidden by the iris, it can be used with little certainty of correct application. At the same time, instead of penetrating the simple structure of the cornea, which bears injury as well as any other struc ture of the body, the instrument pervades the fibrous sclerotic, a structure impatient of in jury and prone to inflammation, punctures the ciliary ligament at the imminent risk of in juring one of the ciliary nerves or even wound ing the long ciliary artery, and finally passes through one of the most vascular parts in the body, the corpus ciliore. The practice appears a signal instance of the influence of education, habit, and authority in setting improvement at defiance. The proofs afforded of the close vicinity of the margin of the pupil to the cap sule of the lens, should remind the surgeon that one of the greatest dangers to be ap prehended in iritis is the adhesion of these two parts, and that one of the first steps in the treatment should be to separate them by the application of belladonna, which, by its pecu ='-^ influence on the pupil, dilates that aper ture, qnd, consequently, brings its margin more Opposite the circumference of the lens and at a greater distance from the prominent central portion..
The aqueous tteimour, although constituting so essential a part or die optical mechanism of the eye, is but small in 'quiantity ; according to Petit not more than four or five grains. Its specific gravity and refractne power scarcely differ from that of water; and according to Bcrzelius, 100 parts contain 98.10 of water, 1.15 of chloruret of soda with a slight trace of alcoholic extract, 0.75 of extractive matter soluble in water only, and a mere trace of albumen. It is perfectly transparent, but is said to be milky in the fcetus.