The capsule retains its transparency after death, but is prone to lose it during life.-The capsule of the lens retains its transparency under the action of acids, of alcohol, and of boiling water, and will resist the putrefactive process for a great length of time: at least, I have frequently found it remain transparent after the lens itself had been completely destroyed by putrefaction, and the centre of the lens is itself very slow to putrefy. But, however difficult it may be to render the capsule opaque after removal, it is rather prone to become so in the living body. An injury, such as laceration or puncture, is there almost sure to be followed sooner or later by a loss of its transparency, and we often see it of a decided dead white. The same also occurs in many cases where the opacity is primarily in the lens itself. After the operation for cataract by the needle, this opaque capsule is a not infrequent source of annoyance to the surgeon, obstructing the access of light to the retina, and demanding removal.

This proneness of the capsule to become opaque only while it continues a part of the living body, seems to shew that, hard and structureless as it appears, it is yet the seat of unceasing nutritional change that its substance is in continual flux; for we can only regard the opacity as a result of depraved nutrition, the new material being laid down in an abnormal form.

In some rare examples, one of which presented itself here during the present summer, minute vessels are developed upon the capsule, probably in lymph previously deposited there as a consequence of inflammation. They are continuous with those of the ciliary processes or adherent iris.

It is interesting to observe that the opacity is usually denser when it takes place in the anterior part of the capsule, than when in the posterior, because of the greater thickness of the former portion. But, besides this, the anterior seems more prone to become opaque than the posterior. When opacity occurs, the capsule usually loses its brittleness and becomes tough. The opacity assumes an irregular figure, in flakes or patches, if the body of the lens remains, and may thus be distinguished from a similar change in the lenticular substance; but the opacity is more uniform if the capsule has been rent and the body of the lens absorbed. The opaque parts may even

* See Case T, in the Appendix.

become so completely altered from their original texture as to be the seat of earthy deposits; but this is rare.

Of the structure of the body of the lens.-If we now turn our attention to the lens itself, that solid transparent mass thus enclosed and protected, we find it to be soft and pulpy in the outer portions, more firm, dense, and glutinous towards the centre, which is distinguished as the nucleus. Not that there is any special plane of division between the nucleus of the lens and its exterior or superficial portions: the change to more and more density is very gradual. No language derived from other objects can adequately describe the precise texture of the lens, as appreciated by the finger, simply because it is not a homogeneous texture, but one highly complicated and peculiar, which it will require some attention to understand.

Fibres of the lens.-The lens is composed of flattish riband-like albuminous fibres, having an average thickness of of an inch, united side by side, so as to form plates, which are placed one within the other, somewhat like the leaves of an onion. The fibres all pass from the front to the back, so that each has two extremities, an anterior and posterior; and a middle part, which is directed towards the side or rim of the lens. In the lens of simplest construction— the spherical or spheroidal lens of many fishes, reptiles, and birdsthe ends of the fibres all meet in the antero-posterior axis; and the surface of such a lens, viewed either before or behind, has the appearance of a globe marked by the lines of longitude passing from pole to pole. The same appearance, too, is seen after removing any number of the layers of fibres down to the centre. The individual fibres are of course narrower at the extremities and broader in the middle; and they would come to quite a point in the axis were it not that their lateral union becomes so intimate as they approach it, that the eye can no longer distinguish them individually, nor the skill of the anatomist isolate them. Moreover, it would appear that those coming from opposite sides do not form a firm junction across the axis, but rather that the axis is occupied by a substance of less density than the fibres themselves; so that, under ordinary circumstances, the lens may be made to break up, and its opposite sides to fall asunder along that line. In the lenses I am now referring to it is not uncommon to find a cup-shaped depression-a kind of crater at each pole; but I have never seen this so large as in the prolatespheroidal lens of the cuttle-fish.

F

Nucleus of the lens.-It is further to be observed that the individual fibres become narrower and denser, as well as more intimately held together, as they approach the centre of the lens; and it is obvious that they must also become shorter and shorter. The degree in which their density augments varies, however, very widely: in the bird, for instance, it is far less than in the fish; so that the lens of the former is soft and pulpy, even to the centre, while the nucleus of the latter is often of almost stony hardness.

What I have now said as to the shape and texture of the lenticular fibres applies in general to the eyes of most animals. These fibres are always narrowest at their ends, shorter and denser towards the centre of the lens. The mode, however, in which their extremities are arranged at the poles, exhibits many very curious modifications, of the use and meaning of which I am not aware that any explanation has hitherto been offered.

Central planes.—The first departure from the simple arrangement already mentioned-in which all the fibres diverge from, and terminate in, the antero-posterior axis of the lens-is met with in some fishes and some mammalia, of which the porpoise is one. Looking at the front of the lens, we see a straight line passing through the pole, and reaching about one-quarter or one-third of the way towards the margin or equator on each side. From this line the fibres diverge in an uniform manner, and passing over the edge, may be traced converging on the opposite surface to a line of similar length passing through the pole, but at right angles to the first,-so that if the one is vertical the other is horizontal. This being so, a moment's consideration will enable you to understand that none of the fibres reach half round the lens-that, for instance, one which starts from the anterior pole (or the centre of the anterior line) cannot reach the posterior pole, but terminates at the extremity of the posterior line; while one which starts from the end of the anterior line is necessarily brought to the posterior pole; and the intermediate ones in a similar manner, according to their position. Now, if we remove the more superficial strata of fibres, we still find the deeper-seated fibres diverging from similar lines, and discover, in fact, that the lines seen on the surfaces are but the edges of planes which penetrate even to the central region of the lens,-these planes being productions or expansions of that axis in which, in the spherical variety of lens, all the fibres meet.

CENTRAL PLANES OF THE LENS.

67

Object of the central planes.-These planes are widest where they appear on the surface of the lens, and are gradually narrower inwards; and those of opposite sides meet, although at right angles, somewhere in the antero-posterior axis, at a point the position of which (or, in other words, the respective depth attained by the planes), is determined by the various curvatures of the opposite surfaces of the lens. But as every fibre has in each plane a point answering to one of its extremities, it follows that the area of the two planes must in all probability be equal, and therefore that where one plane passes from the surface more deeply into the lens, the other must extend from the pole more widely towards the margin.

It certainly appears to me that the expansion of the axis into the planes now described, and the concomitant complexity of the arrangement of the fibrous constituents of the lens, are designed to furnish the mechanical means of modifying the curvature of the surfaces of the lens. It would appear that by carrying the points of termination of many of the fibres away from the antero-posterior axis, and towards the margin of the lens, this axis is shortened, and the surfaces are rendered flatter in the middle part than towards the edge; are made to resemble ellipsoids of revolution round the lesser axis, rather than spheres; and this in a degree proportionate to the extension of the central planes towards the circumference. Thus we have the central planes wider in front than behind, in correspondence with the greater flatness of the anterior surface.

If we pass to the examination of other lenses, further modifications of the axial planes, and consequently of the arrangement of the fibres, are met with. For example, in some of the cetacea I have found the planes to bifurcate irregularly, and to a variable extent, towards the margin of the lens-a disposition not, I believe, before observed; but the most elegant arrangement is certainly that well known one of the mammalia in general, in which three equidistant planes diverge from the axis, those of the front and back holding intermediate positions, precisely as in the more simple case already described.

Their complexity in the adult human lens.-But of all the specimens that have come under my own observation, those of the adult human lens have presented the greatest multiplication or subdivision of these planes; for while our own lens adheres to the ordinary mammalian type in possessing a triple divergence from the pole, each of the three planes is almost immediately branched, if I may use

the term, and this not once only, but twice or more,- -so that instead of three segments we have as many as twelve or sixteen, the numbers being irregular no less than the course, direction, and extent of each.

A. Division of central planes as seen on

Fig. 11, A, copied with accuracy from an adult human lens, will convey a better idea of the arrangement of these planes than mere words can express; and if you will endeavour to picture the opposite surface as if seen through this one, and intersected with a somewhat similar radiation of planes, placed intermediately to these and receiving the opposite ends of the fibres, you will understand the extraordinary intricacy of the construction of this organ in our own eye.

Their simplicity in the human fœtus.-I may mention in this place an interesting fact which I noticed in comparing the fibres of the fœtal and adult lens in the human subject. It is this-that as development proceeds, and the

posterior surface of an adult human lens becomes wider and flatter,

lens.

B. Same from the fœtus of nine months.

the central planes extend themselves further and further from

the axis, and at the same time branch again and again, so as to multiply the segments, into which they divide the lens. See fig. 11, B.

From this it may be inferred that the multiplication of the mesial planes outwards is a process necessary to the expansion and flattening of the organ, and takes place by the deposition of new fibres on the old; and also, that even in the adult lens the planes remain simply tripartite in the nucleus, being only multiplied in the more superficial layers.

Having thus endeavoured to convey to you some general idea of