From the preceding observations respecting the ciliary processes of the vitreous humour, it may justly be inferred that the ciliary pro cesses of the choroid, and these ciliary pro cesses of the vitreous humour, are of the same nature, differing only in those of the choroid receiving red blood, while those of the vitreous humour receive a transparent fluid by their bloodvessels. The adaptation of these two circles of folds to each other appears to be a most beautiful example of mechanical con struction occurring in soft parts: it is a species of dovetailing of the one structure into the other, by which an intimate union is secured between one part of considerable strength and another of extreme delicacy. A connexion equally perfect is established between the ex ternal surface of the choroid at its margin, and the corresponding margin of the sclerotic, by means of the ciliary ligament; in fact, with out these two provisions of ciliary ligament and ciliary processes, and their application between the sclerotic, choroid, and vitreous humour, the chambers of the eye must be imperfectly constructed, and the optical me chanism of the organ defective. It is the mechanical bond between these dissimilar parts which perfects the chamber of aqueous humour, and prevents that fluid from escaping, either between the sclerotic and choroid, or between the choroid and vitreous humour.
()I' the crystalline lens.-1 t has been al ready stated, that there is a double convex lens within the sphere of the eye, at a short distance behind the external lens or cornea. This is the crystalline lens or crystalline humour, which gives additional convergence to the rays of light transmitted through the pupil. It is placed in a depression, formed for its reception on the anterior, compressed, or truncated portion of the vitreous humour, where that body approaches the back of the iris, and constitutes part of the boundaries of the posterior chamber of the aqueous humour. In this depression it adheres firmly to the hya loid membranes, and from the vessels of that structure derives its nutriment.
This double convex lens does not present the same curvature on both surfaces, the anterior being less curved than the posterior, in the ratio of about 4 to 3. Attempts have been made to determine with accuracy the nature of these curvatures, first by Petit, and subsequently by Wintringhani, Chossat, and others. The re cults of the numerous experiments of Petit lead to the conclusion, that the anterior curvature is that of a portion of a sphere from six to seven lines and a half in diameter, the posterior that of a sphere of from five to six lines and a quarter. From the same source it appears that the dia meter is from four lines to four lines and a half, the axis or thickness about two lines, and the weight three or four grains. I am, however, inclined to agree with the observation of Porter field, that, " as it is scarce possible to measure the crystalline and the other parts of the eye with that exactness that may be depended on, all nice calculations founded on such measures must be fallacious and uncertain, and, therefore, should, for the most part, be looked on rather as illustrations than strict demonstrations of the points in question." The method by which Petit arrived at these results must render them of doubtful value, the curvatures having been determined by the application of brass plates cut to the requisite form. The results of Chossat's experiments, conducted with great care, and with the assistance of the megascope, are thus stated by Mr. Lloyd in his Treatise on Optics: "This author has found that the cornea of the eye of the ox is an ellipsoid of revolution round the greater axis, this axis being inclined inwards about 10°. The ratio of the major axis to the distance between the foci in the generating ellipse he found to be 1.3 ; and this agreeing very nearly with 1.337, the index of refraction of the aqueous humour, it follows that parallel rays will be refracted to a focus, by the surface of this humour, with mathemathical accuracy. The same author found likewise that the two surfaces of the crystalline lens are ellip soids of revolution round the lesser axis; and it is somewhat remarkable thatthe axes of these sur faces do not coincide in direction either with each other, or with the axis of the cornea, these axes being both inclined outwards, and containing with each other, in the horizontal section in which they lie, an angle of about 5°." It must not be forgotten that these observations apply to the crystalline of the ox, not to that of into, and also that, as Chossat himself admits, the evaporation of the fluid part of the lens, or the absorption or imbibition of the water in which it is immersed, may materially alter the curva ture. 1 cannot myself believe it possible to separate a fresh lens in its capsule perfectly from the hyaloid membrane without injuring its structure, and endangering an alteration in its form. llaller states that Kepler considered the anterior convexity to approach to a sphe roid, and the posterior to a hyperbolic cone. Wintringham states the results of his inquiries as to this matter as follows 1—" In order to take the dimensions of the eye of an ox, I placed it on a horizontal board and applied three moveable silks, which were kept extended by small plummets, so as to be exact tangents to the arch of the cornea, as well at each can thus, as at the vertex; then applying a very exactly divided scale, I found that the chord of the cornea was equal to 1.05 of an inch, the
versed sine of this chord to be 0.29, and con sequently the radius of the cornea was equal to 0.620215 of an inch. I then carefully took off the cornea, and replaced the eye as before, and found, by applying one of the threads as a tan gent to the vertex of the crystalline, that the distance between this and the vertex of the cor nea was 0.355 of an inch. Afterwards I took the crystalline out without injuring its figure, or displacing the capsula, and then applying the threads to each surface of this humour, as was done before to the arch of the cornea, I found that the chord of the crystalline was 0.74 of an inch, and its versed sine, with respect to the anterior surface, to be 0.189 of an inch, and consequently the radius of this surface was 0.45665 of the same. In like manner the versed sine to the same chord, with respect to the posterior surface of the crystalline, I found to be equal to 0.38845 of an inch. Lastly, I found the axis of the crystalline and that of the whole eye from the cornea to the retina to be 0.574, 2.21 respectively.' Whatever doubts may be entertained respecting the accuracy of the measurements of the lens, there can be none that the form is different at different periods of life, in the human subject. It also appears to differ in different individuals at the same period of life, and probably the curvature is not the same in both eyes. In other animals the dif ference in form is most remarkable. In the human foetus, even up to the ninth month, it is almost spherical. Petit states that be found the anterior curvature in a fetus of seven months, a portion of a sphere of three lines diameter, and the posterior of two and a half, and the same in a new-born infant. In an in fant eight days old, the anterior convexity was a portion of a sphere of four lines, and the posterior of three. All anatomists concur in considering the lens to approach more to a sphere at this period. In childhood the curva tures still continue much greater than in ad vanced life; from ten to twenty probably de crease, and from that period to forty, forty-five, or fifty, remain stationary, when they become much less ; being, according to the tables of Petit, portions of spheres from seven to even twelve lines in diameter, and on the posterior of six or eight. Every day's observation proves that the lens becomes flattened, and its curva tures diminished as persons advance in life. It is seen in dissection, when extracted by opera tion, and even during life; the distance between its anterior surface and the back of the iris be ing so great in some old persons, that the sha dow of the pupil may be seen upon it, while at an earlier period it actually touches that part of the membrane. This diminution of the curva tures of the lens commences about the age of forty-five. Petit found the anterior convexity varying from a sphere of about seven to twelve lines diameter, and the posterior from five to eight in persons from fifty to sixty-five years of age. The alteration in power of adaptation, and the indistinctness of vision of near objects which takes place at this period, is probably to be attributed to this cause, although a diminu tion of the muscular power of the iris, and con sequent inactivity of the pupil, may contribute to the defect. It is also to be recollected that the density of the lens is much increased at this period, and that the young person whose lens presents greater curvatures does not require concave glasses, as the old person requires con vex ones. The state of the eye, after the re moval of the lens by operation for cataract, proves that it is a part of the organ essentially necessary for correct vision. When the eye is in other respects perfect, without any shred of opaque capsule,any irregularity or adhesion of the pupil, or any alteration in the curvature of the cornea, as in young persons who have had the lens properly broken up with a fine needle through the cornea, vision is so good for distant objects, that such persons are able to pursue their common occupations, and walk with safety through crowded streets, but they require the use of a convex lens, of from three and a half to five inches focus, for reading or vision of near ; old persons, however, generally require convex glasses on all occasions after the removal of the lens. That the curvatures of the lens are fre quently different in different individuals may he inferred from the frequency of short sight, or defective power of adaptation, not attributa ble to any peculiarity of the cornea. Petit states that he found lenses of which the two convexities were equal, and others of which the anterior was greater than the posterior, and more than once, one more convex on its ante rior surface in one eye, while that in the other eye was in a natural state. lie also occasion ally found the lens as convex in the advanced period of life as in youth. I have repeatedly observed the perfection of vision and power of adaptation much greater in one eye than the other in the same individual, without any defect of the cornea, pupil, or retina; and occasionally have found young persons requiring the com mon convex glasses used by persons advanced in life, and old persons becoming near-sighted, and requiring concaves. The annexed letters chew the difference of curvature at the different periods of life, as represented by Sommerring. A is the lens of the foetus; B, that of a child of six years of age ; and C, that of an adult.