Of the sclerotic as has been stated, constitutes, with the transpa• rent cornea, the external case upon which the integrity of the more delicate inter nal parts of the organ depends, otherwise in capable of preserving their precise relations to each other : without such support the compo nent structures must fall to pieces, or be crushed by external pressure. The name is derived from the Greek alangool, and it has also been called cornea and cornea opaca in contradistinc tion to the true or transparent cornea, a structure to which it bears no resemblance whatsoever ; it is the same animal material which exists in all parts of the body where strength with flexi bility is required, the material which in modern times has been denominated fibrous mem brane. When carefully freed from all ex traneous matter by clipping with a pair of scissors under water, it presents the brilliant silvery-white appearance so characteristic of the fibrous membranes. The white streaks which give the fibrous appearance appear ar ranged concentrically as the lines on imper fectly polished metallic surfaces. It is inelastic as other fibrous membranes, and so strong that it does not tear or yield unless exposed to the greatest violence. Although penetrated by the vessels going into and returning from the in ternal parts of the eye, it does not appear to have much more red blood circulating through its texture than other tendinous expansions distinguished for their whiteness. The vas cularity of the anterior part, however, where it is exposed in the living body, constituting the tunica albuginea, or white of the eye, is different from that of the rest of the mem brane. The four straight muscles are pene trated by small branches of the ophthalmic artery, the delicate ramifications of which con verge to the circumference of the cornea, for the nutrition of which membrane they appear to be destined. In the natural state they can scarcely be detected, but when enlarged by in flammation, present a remarkable appearance, considered by practical writers one of the most characteristic symptoms of inflammation of the eyeball, or, as it is called, iritis. They then appear as numerous distinct vessels, and as they approach the margin of the cornea, become so minute and subdivided, that they can no longer be distinguished as separate vessels, but merely present a uniform red tint, described as a pink zone. The colour of this inflammatory vascula rity is also characteristic. Whether from the vessels being more arterial than venous, or from their distribution in so white a'structure, they present a brilliant pink appearance very different from the deep red of conjunctival in flammation, which often enables the practi tioner to pronounce an opinion as to the nature of the disease before he makes a close examin ation.
The inner surface of the sclerotic where it is in contact with the choroid, does not present the same brilliant silver-white appearance that it does externally, being stained with the black colouring matter ; it is also obscured by a thin layer of cellular membrane, by means of which it is united to the external surface of the cho roid.* This layer of cellular membrane was described by Le Cat, and more particularly by Zion, as a distinct membrane, and considered to be a continuation of the pia miter ; it is, how ever, obviously nothing more than the connect ing material applied here as in other parts of the body where union is requisite.
The thickness of the sclerotic is greater in the bottom of the eye than at its anterior part, where it is so thin that it allows the black colour of the choroid to appear through it, giving to this part of the eye a blue tint, particularly remark able in young persons of delicate frame. The at tachments of the four straight muscles, how ever, appear to increase the thickness in this situation ; but that there is no general thick ening in this part from this cause is proved by the thinness of the membrane in the inter vals between and beneath these tendons. The consequence of this greater thinness of the membrane anteriorly is, that when the eyeball is ruptured by a blow, the laceration takes place at a short distance from the cornea. In animals in whom the eyeball deviates much from a true sphere, as in the horse, ox, sheep, and above all, in the whale, the sclerotic is much thicker posteriorly than anteriorly, being in the latter animal from three quarters to an inch in thickness, while it is not more than a line at its junction with the cornea. The rea
son for the existence of this provision is, that the form of the perfect sphere is preserved by the uniform resistance of the contents, but when these contents are spherical in one part, and flattened in another, the external case must pos sess strength sufficient to preserve this irregu larity of form. It is remarkable that this strength is conferred in the class maminalia by giving to the sclerotic increase of thickness, the fibrous structure remaining nearly the same in its nature, while in birds, reptiles, and fishes, the requisite strength is derived from the pre sence of a cartilaginous cup or portion of sphere, disposed within a very thin fibrous sclerotic. This cartilaginous sclerotic, as it is often called in the books, exists, as far as I have been able to ascertain, in these three classes, and is in some individuals very remarkable. In birds it is thin and flexible, giving a degree of elasticity, which distinguishes the eyeball in this class. In fishes, as has been observed by Cuvier and others, the cartilage is always present, and is particularly thick in the sturgeon ; it is even osseous in some, as the sea-bream, from the eye of which animal I have often obtained it in the form of a hard crust by putrefactive maceration. Among the reptiles the turtle presents a good example of this structure. Where the deviation from the spherical form is very great, as in birds, additional provision is made to sustain the form of the organ. This consists of a series of small osseous plates ar ranged in a circle round the margin of the cor nea, lapping over each other at the edges, and intimately connected with the fibrous and car tilaginous layers of the sclerotic. A similar provision exists in the turtle, and also in the chameleon, and many other lizards, but not perhaps so neatly and perfectly arranged as in birds. It is found in the great fossil reptiles Ichthyosaurus and Plesiosaurus.
The sclerotic, like other fibrous membranes, being inelastic and unyielding, does not be come stretched when fluids accumulate in the eyeball in consequence of inflammation, or in other words, the eyeball does not become en larged from effusion of serum or secretion of purulent matter into its chambers. To this probably may be attributed the intolerable torture and sense of tension experienced when the eyeball suppurates, as well as the severe pain extending to the temple in some forms 9f inflammation. The pain in such cases must not, however, be wholly attributed to this dis tension of an unyielding membrane. The fibrous membranes in general, when affected by rheumatic or arthritic inflammation, become acutely sensible, and the cause of much suffer ing ; and the sclerotic, when similarly affected, acquires the same description of painful sen sibility, apparently independent of distension from effusion. In certain forms of inflam mation and other morbid changes of the eyeball, the sclerotic appears to yield to distension, as in scrofulous inflammation and hydroplithalmia ; but this is not a mechanical stretching, but an alteration in structure at tended with a thinning of the membrane, and consequent alteration in the shape of the globe. It appears that the cornea and sclerotic are peculiarly, if not in many instances almost ex clusively, the seat of the disease in chronic scrofulous inflammation of the eyeball. This inference may, I think, be justly drawn from the fact, that in such cases the sclerotic becomes so much thinned that the dark choroid projects in the form of a tumour, and the eye loses its spherical form ; yet the pupil remains regular, the lens transparent, and the retina sensible to light. When the cornea is destroyed by slough or ulceration in severe ophthalmia, allowing the lens and more or less of the vitreous hu mour to escape, the sclerotic does not accom modate itself to the diminished contents by a uniform contraction, but merely falls in ; and when the eye has been completely emptied, it is found many years after the injury folded up into a small irregular mass in the bottom of the orbit. When the organization of the eye is completely destroyed by idiopathic, rheumatic, or syphilitic inflammation, the sclerotic becomes flaccid, and the whole eyeball soft, allowing the contraction of the four straight muscles to produce corresponding depressions, and thus convert the sphere into a form somewhat cu bical.