regulating the quantity of light admitted to the retina. To allow of the movements of the iris, the transparent medium across which it is extended, and which fills the concavity of the cornea, is a fluid one, the aqueous humor. Behind the aqueous humor and the iris is the crystalline lens, the most solid and highly refracting of the media, imbedded in the front of the third humor, the vitreous, which itself occupies nearly four-fifths of the globe, and fills and supports the hollow of the retina. To hold the crystalline in its place, the choroid is fixed to the outer case at the junction of the sclerotica and cornea and sends inwards a circle of folds, the ciliary processes, which impress, and fasten themselves to, the vitreous humor all round the lens.

It is unnecessary that I should speak at present, even in general terms, of the mobility of the eye, of its outer protective appendages or of the source of its supply of blood. The limits to which I am confined oblige me to proceed at once to those points in the anatomy of the globe which more immediately concern our present object. And, first, of the size and shape of the eyeball.

Shape of the eyeball.—The human eye, when carefully separated by dissection from the muscles implanted in it, and from the surrounding fat and areolar tissue, is seen to be about spherical in shape, with this exception-that the cornea which forms the front clear part, whereby the light has access to the interior, bulges somewhat beyond the rest, is more convex, and is the segment of a smaller sphere. The sphere of the sclerotica, however, is not absolutely geometrically true in all cases, or even generally. I have found it many times slightly spheroidal, with the longer axis sometimes transverse, sometimes longitudinal; and even other and more irregular departures from the exact spherical shape occur, the lateral regions occasionally presenting trivial swellings, chiefly between the muscles, and not affecting the integrity of the organ as an optical instrument. Behind, where the retina is spread out, the sclerotica is thickest and stiffest, most completely retaining its proper curvature even after it has been cut into small pieces; and it is interesting to observe how carefully the exactness of its curvature in the posterior region has been provided for in the construction of the eye generally in the animal series: in some instances by extraordinary thickness and density of the fibrous tissue; in others by the development of very thick or highly elastic cartilage, and even bone, in its stead.

SIZE AND TENSION OF THE EYEBALL.

Size of the eyeball.-The size of the adult human eye varies within certain limits, as might have been expected. Nevertheless, as it is a part which, by reason of its complex mechanism, and the peculiarity of some of the textures it comprises, attains its complete development, like the internal ear, before most other organs of the body have reached their adult condition, these limits are very confined, if we except instances amounting to disease. Of many measurements which I have made, the general result is that the diameter of the sclerotica is from seveneighths of an inch to an inch.

The transparent cornea forms by its anterior surface a portion of a sphere, the diameter of which is from 2 to 3 of an inch (that is, of usually less than two-thirds of an inch), and it often happens that the surface of the globe recedes, or is depressed, very superficially near the line of junction of the sclerotica and cornea. The cornea forms from one-sixth to one-seventh of the horizontal circumference of the whole globe.

In considering the size of the eye, I should guard you against judging of it in any degree by the size of the aperture of the eyelids; the latter, indeed, is that which most governs the apparent size of the ball, and is also of much importance to the practical surgeon, as enabling him more or less readily to manipulate in his operations on parts within the lids. Moreover, the cornea is often apparently smaller than it really is, in consequence of the formation of a ring of opacity ciose to its border in the declining stages of life.

Tension in health and disease.-The eyeball has naturally a certain tension, arising from the due repletion of the outer case with the tissues contained within. It gives a tight or resisting feel to the finger applied upon it, and the exact degree of this tension belonging to the healthy state it is essential for you to know, both that you may be aware of the resistance your instruments will encounter, and also that you may be able rightly to appreciate by the touch the departures from the healthy standard of tension which occur in the course of several diseases. When disorganizing processes have occurred in the interior, the eyeball frequently becomes soft, at other times hard, although, perhaps, the finger alone can inform you of these evidences of the impaired nutrition of the organ. Again, in inflammations of an acute kind attacking the globe, an unnatural hardness is perceptible on pressure, usually accompanied by that dull sickening pain which attends distension of the fibrous tissues, and referrible in this case, I

The un

suppose, in a great measure to the sclerotica and cornea. yielding nature of these coats occasions all internal distending forcesas vascular engorgements, fibrinous or purulent effusions, if their accession has been sudden and rapid,—to react in the way of counterpressure on the parts within, impairing their functions in the first instance, and soon irrecoverably destroying them, if allowed to continue unchecked. We have analogous phenomena in the case of other organs enveloped, like the eye, in a tunic incapable of hasty dilatation.

OF THE SCLEROTICA.

To proceed now to a more particular account of the sclerotica and And first of the sclerotica.

cornea.

Structure of the sclerotica.-The sclerotica consists of a very dense and intricate interlacement of white fibrous tissue. The surface is not glistening, like most other examples of the tissue, but dead white, by reason of the working up of its fibres in an irregular way, into a web which exhibits little indication of their course. In tendons, in fasciæ, even in the dura mater, a silvery lustre results from the parallel course of contiguous fibres, and the small creases occurring on these confer a satiny surface, which is extremely characteristic and beautiful; but this is not the case with the sclerotica. Those parts of it which are exposed to view, as the white of the eye, have no lustre of their own: the brilliant reflection of light is rather from the moist surface of the investing conjunctiva, set off, it is true, by the opaque white foil of this tunic behind. The fibres of the sclerotica, when unravelled from one another, and examined in minute portions under a high magnifying power, do not quite resemble those of tendon and fascia: they are more straight and stiff, less wavy, less connected they also tear and break more easily, so that you cannot run them out into such long shreds as in the case of tendon or ligament. Nevertheless, they swell out and become semi-transparent with acetic acid or the caustic alkalies, just as ordinary white fibrous tissue. You can best see the characteristic differences I have now alluded to in the posterior part of the sclerotica of the ox or sheep. This much resembles the thick coat of the air-bladder of the sturgeon, which I had once an opportunity of examining in the fresh state, and it is possible that it might even be made to yield isinglass like that

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IMPLANTATION OF THE RECTI MUSCLES.

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structure. There is also a good deal of delicate yellow elastic tissue intermixed with the white in the sclerotica.

But although it cannot be said that the fibres of the sclerotica are arranged with regularity, yet they appear to have a more or less determinate direction from behind forwards in the hinder and middle portion; and we may also sometimes observe on the inner surface, after the choroid has been removed, an appearance of arching fibres, having their convexities turned forwards; and, moreover, the anterior part usually presents a different arrangement on its outer and inner surfaces, the fibres of the former being more circular, following somewhat the border of the cornea, especially over the insertion of the recti muscles; while those of the latter are more obviously directed forwards.

Mode of implantation of the tendons of the recti muscles.With reference to this subject, I may allude to the mode of implantation of the tendons of the recti muscles into the sclerotica. These, as is well known, become flattened and expanded somewhat before joining the sclerotica; and I believe it is a common opinion that they join side to side, and spread out as a kind of external investment to the front of the sclerotica, advancing up to the cornea, and constituting the white tunic which is visible between the lids. I do not find, however, that this description, as regards the human eye, agrees with nature. On the contrary, the tendons of the recti appear to be truly implanted into the sclerotica, penetrating its substance, so as to be buried from view, and leaving its exterior layers exposed under the conjunctiva. I have several times traced the continuation of these tendons for a considerable way forwards in the middle substance of the sclerotica, to a certain extent making a division of it into an inner and outer layer, and gradually becoming lost to view, as they break up into laminæ, and blend with the neighbouring structure very near the margin of the cornea.

Thickness of the sclerotica at different parts.-It may not seem very important in a practical point of view to inquire into the relative thickness of the sclerotica at different parts of its extent. It is interesting, however, in a physiological sense, especially with reference to comparative anatomy, and is really not without its practical bearings, since we find the effects of certain morbid actions to be limited or otherwise modified in correspondence with it. The sclerotica is thickest behind, for the support of the retina, and for the preser

vation of its due curvature at the most material spot. Around the foramen by which the optic nerve enters to join the retina, and which is near the bottom of the eye, the sclerotica is about 1-25th of an inch thick. Hence it becomes thinner forwards as far as to a quarter of an inch behind the cornea, where it is only about 1-40th of an inch thick. From this line to the cornea it again increases in strength, and is from 1-30th to 1-35th of an inch thick, so as to be able to give greater support to some of the internal parts, which I shall have to speak of in a subsequent lecture. In the monkey, and in many of the smaller mammalia, I have found this front part the thickest. of all.

The recti muscles being inserted as described, groove the sclerotica before entering it, and hence this membrane is rendered very thin mmediately behind their insertion; hardly, in fact, being more than 1-60th of an inch thick in those parts. These being, therefore, the weakest parts, are those which might be expected to yield earliest under any inward distending force; and accordingly I have observed that abscesses of the eyeball are prone to point in these situations.

While upon the subject of the insertion of the recti muscles, it may be mentioned that, in cases where the contents of the globe are diminished in quantity, as a result of pre-existing inflammation of a destructive kind, and the eye consequently shrinks to a smaller size, the softness of the organ allows the recti muscles to impress it in the lines of their transit, and to bulge it in their intervals, thus pinching it, as it were, into a quadrangular shape. In such examples the thinner parts of the sclerotica under the tendons of the recti, being pressed upon by those tendons, and unsupported from within, are sunk in or flattened.

OF THE CORNEA..

We will now turn our attention in a more particular manner to the transparent portion of the outer case, the cornea, a part of which it would be difficult to exaggerate the importance in reference to the operative surgery, or the pathology of the eyeball, and which can hardly fail to attract your interest in a high degree, however imperfect my description of it may prove.

I shall not stop to inquire what light comparative anatomy, or the