ADJUSTMENT OF THE EYE TO DISTANCE.

59

into the canal; forms, in fact, its inner wall, and is free, as I before stated. I say free, and it is so in the main, but I have several times seen isolated elastic fibres of the suspensory ligament passing from the ciliary processes to the very margin of the lens, and there implanted. The width of the canal of Petit is about 1-10th or 1-12th of an inch.*

I find that I shall not be able to give a full description of the crystalline lens in the remaining part of the time allotted to the present lecture, and I shall therefore defer that subject, and now merely detain you for a few moments with some remarks on one part of the mechanism which the eye appears to contain for its adjustment to distinct vision at different distances.

Adjustment of the eye to vision of objects at different distances. -You are aware that when we look at a distant object, the intervening ones at a less distance appear indistinct, but that when by a voluntary effort we render near objects distinct, distant ones become confused. If we use both eyes, the distant or near object which is indistinct, is at the same time double, but, with one eye, it is merely confused. In the former case the convergence of the optic axes can only be towards one of the objects at once, and the image of the other consequently falls on non-corresponding parts of the two retina, and is seen double. But in the latter case it is clear that the confusion arises simply from the rays from one of the objects not coming to an exact focus in the retina, while those from the other object meet exactly there, and form a distinct image, to which the attention is at the same time directed. This results from optical laws; and hence the necessity of a power in the eye to adjust its dioptric apparatus, so that objects at various distances may be brought successively into distinct view.

To enter upon a discussion of the several modes in which it has been supposed that this adjustment is or might be effected, would be now impossible. It has been thought that the antero-posterior axis of the eye is capable of elongation,-that the cornea, or lens, may become more convex, or that the lens may be advanced towards the

* For some cases in which the attachments of the lens became destroyed during life, especially for a very remarkable instance in which the lens became loosened from its connexions, except at one point, and yet retained its transparency, see Appendix, Cases O, P, Q, and R.

cornea by a kind of erection of the iris or ciliary processes during near vision. It is manifest that numerous purely optical considerations must occur in the examination of such a question, but on the present occasion I am concerned rather with such arguments as the anatomical construction of the eye may contribute, towards the solution of the difficulty; and I may be allowed to insist on the extreme and primary importance of such arguments in all similar questions, provided they are grounded on a correct appreciation of the facts.

We have, then, it appears, within the globe of the eye a structure which is in all probability muscular, and which is undeniably so in certain animals. It has the arrangement of a muscle, very much the structure of a muscle, and is largely supplied with nerves, which are in great part derived from a motor source-the third pair. This muscle arises, or has its most fixed attachment, at the junction of the sclerotica and cornea, as much in front of the lens as is possible consistently with the preservation of the transparency of the cornea. That it may act more freely, a canal, the circular sinus, is interposed between its origin and the portion of the sclerotica which it lies against. Beyond this point it is hardly at all attached to the sclerotica, over which its fibres may be supposed to move in contraction, but it covers, and is inserted into, the anterior one-eighth of an inch of the choroid membrane, which is in this part tougher and firmer than elsewhere, and united in a very special manner to the lens by the ciliary processes, through the medium of a firm tough membrane, and of a strong elastic fibrous membrane proceeding from it to the margin of the lens. And yet not quite to the margin, for an elegant arrangement exists, the canal of Petit, by which traction is made, not on the vitreous around the lens, nor on the edge of the lens itself, so much as on its anterior surface. I confess it seems to me very difficult to doubt that this complicated system of parts is intended to advance the lens towards the cornea, so as to bring forwards, up to the retina, the focus of a near object, which would otherwise fall behind the nervous sheet. It is possible, also, I think, from the peculiar direction taken by the ciliary muscle, that it may compress the front of the vitreous, and thus help to throw forwards the lens.

LECTURE IV.

OF THE CRYSTALLINE LENS. Position, shape, size-Anterior chamber small in infancy-Occasional consequence of this. Capsule of the Lens-Experiment illustrating its endosmodic power and its elasticity-Thickness not uniform-Proneness to opacity during life-Characters of the opacity. Body of the lens-Its fibres and lamina-Nucleus-Central planes-their use-their complexity in the human lens-Mode of union of the fibresUse of the toothed margins-Intracapsular cells-The "liquor Morgagni" a result of disease or post-mortem change-The "capsule of the aqueous humor" does not exist-Remarks on some appearances of cataractous lenses.

GENTLEMEN,-We proceed to-day with the consideration of the structure of the crystalline lens and its capsule; a subject not more interesting to the anatomist than to the ophthalmic surgeon, on account of the large share of his attention which that important and common disease, the cataract, must always engross.

The crystalline lens, you will remember, is placed at the front of the vitreous body behind the iris, and is held in place there chiefly by a special fibro-membranous suspensory apparatus passing between its capsule and the ciliary processes, but partly by its adhesion to that portion of the vitreous body which is hollowed out to receive it.

Size and shape.-The lens is an almost perfectly transparent structure, about one-third of an inch wide, and one-sixth of an inch thick, more convex behind than in front. It has been doubted whether the curves of the lens are spherical or spheroidal; a question of much interest with reference to the corrections of an optical nature of which the eye is the seat, but not strictly bearing on the object before us in these lectures. The most accurate admeasurements, however, of the mammalian lens, which are those of Chessat, make it

probable that the figure of the human lens is like that generated by an ellipse revolving round its lesser axis, the curvature being greater for the lateral than for the central parts.

The lens in early life is soft and nearly spherical, and grows larger and flatter with age, as well as harder, and somewhat amber-coloured. These circumstances should be remembered with reference to the diseases of the part at the several periods of life. The globular shape of the infant's lens renders the aqueous chambers small, and brings the iris almost into contact with the cornea; but in the adult the iris is usually not at all thrown forwards by the prominent centre of the lens. Nevertheless, in persons of full or declining age, who are the common subjects of hard cataract, the surgeon is accustomed to meet with very varying dimensions of the anterior chamber. This, however, depends rather on variation in the size of the lens than in its shape, and sometimes a prominent iris betokens an enlarged vitreous body, or chronic engorgement of the ciliary body of the choroid.

In consequence of the prominence of the lens in infancy, it sometimes acquires a minute opacity in the very centre or most prominent point of its anterior surface, from coming in contact with the cornea, where this membrane is inflamed in cases of purulent ophthalm a. There is often a corresponding speck on the posterior surface of the cornea, precisely opposite.*

Of the capsule of the lens.-The lens is enclosed in a capsule of perfectly transparent, homogeneous, and very elastic membrane-a part that should engage the special study of every one who proposes to operate on the eye. It is an entire unbroken layer, separating the lens from all that surrounds it, but very permeable by fluids, and, therefore, the medium through which the nutrition of the lens is carried on. Its elasticity, which is one of its most remarkable properties, is evinced by a curious experiment which presented itself to me accidentally when I was occupied in a series of researches into the anatomy of these parts. When removed from the eye, and placed in water, the lens imbibes fluid through its capsule, which thereby becomes distended and separated from the contained lens, being raised in the form of a vesicle. If it be taken from the water, and punctured with a needle, the fluid is ejected with violence by the resilience of the distended capsule, which instantly contracts to its former

* See Cases S, in the Appendix.

bulk, and grasps the lens closely. The capsule is also very brittle, is easily torn in any direction when once a breach is made in it, and yet very tough, so as to offer considerable resistance to a blunt instrument which may be thrust against it.

We see these points exemplified in the operations for cataract and artificial pupil. In the former, when the sharp-pointed needle touches the capsule (provided the membrane be sound), it enters and tears it with the utmost facility; while in the latter, the blunt hook often used to engage and draw aside the pupillary margin of the iris seldom does any injury to the capsule, though it must almost always touch it, and that sometimes somewhat rudely, in spite of the operator's caution. When ruptured, the capsule rolls up at the edges, whatever the direction of the laceration, and it is curious that the outer or convex surface always lies innermost in the roll, so that, like the posterior elastic lamina of the cornea, which it nearly resembles in other respects, it appears to be developed or laid down in a curve contrary to that which its elasticity inclines it to assume.* It is a hard and dense structure, and determines the exact outline of the lens.

Thickness of the capsule not uniform.-The thickness of the capsule of the lens is different in different parts: particularly it is thicker in front than behind. This I alluded to in the last lecture, in speaking of the suspensory ligament or zonule; but I shall here repeat it, because of its importance. The anterior part of the capsule in the greater portion of its extent, in all its central region, and as far outwards as to within one-sixteenth of an inch of its margin, where the suspensory ligament is attached, is four or five times thicker than the posterior part. The diminution in thickness commences rather suddenly at the attachment of the zonule, and continues gradually as you proceed over the border to the posterior surface, where the minimum thickness is soon attained. This I have ascertained by careful examination. I need hardly point out to you how a knowledge of this fact may help us to understand some of the morbid processes met with in this part, and, by indicating with precision the direction in which lies the chief strength of the support of the lens in its position, may aid the surgeon, and give him confidence in certain of his nice and delicate manipulations.

*For a post-mortem examination of an eye in which the operation of solution had been performed, see Case T, in the Appendix,