CHAPTER I. Remarks on the Anatomy of the Eye-Mechanism of the Accommodation of the Eye. ANATOMY OF THE EYE. FIG. 1. a Orbito-Ocular Sheath.-The eyeball is encased in a CAPSULE fibrous sheath, which commences at the apex of the OF TENON. orbit, and embracing the optic nerve, passes forwards and becomes interwoven with the sclerotic a few lines behind the margin of the cornea. This sheath is known as the orbito-ocular sheath, or capsule of Tenon. It is perforated by the tendons of the obliqui muscles near the equator of the eye, and anteriorly by the tendons of the recti, with which it becomes incorporated (Fig. 1); so that the posterior surface of the globe of the eye glides over the anterior surface of the orbito-ocular capsule, like the head of the femur in the acetabulum. These movements are facilitated by a kind of filamentous tissue, bearing a strong resemblance d с a, b, c, d, sections of the recti to a serous membrane. When muscles; e,e,capsule of Tenon; these relations are altered by Tenon having been removed; Facilitates motion. there results a kind of dislocation of the eye. In ex- Preserved tirpation of the eye, if care be taken not to injure the in extirpa barrier formed by this fibrous membrane, the operation eye. B tion of the Importance of, in operations for is far less dangerous than if it is perforated, because then the soft parts of the orbit become inflamed and suppurate, and may even propagate their inflammation and suppuration to the cranial cavity.* The connexion of the capsule of Tenon with the tendons of the recti muscles has an important bearing on strabismus. the operation of tenotomy for the cure of diplopia. Evidently, if the tendons of the muscles only are divided close to their insertion into the sclerotic, the processes given off from them to the capsule of Tenon will prevent the tendons from suffering too great a retraction, and by fixing the divided ends of the muscles, will allow of their forming adhesions to the sclerotic near their normal point of insertion, often a matter of the first consideration in operations for the cure of strabismus. Form of the eyeball. THE SCLE- THE OPTIC NERVE. If the eyeball is carefully separated from its attachments, it will be found to be nearly spherical; the cornea, being the segment of a smaller sphere, is more convex than any other portion. The eyeball varies in size in different individuals-its mean diameter being about seven-eighths of an inch. The Sclerotic is the most external of the proper tunics of the eyeball, forming a dense, opaque, fibrous casing, which gives shape and support to the delicate structures within; its texture is modified anteriorly, where it forms the cornea, so as to become transparent and admit the passage of light to the interior. The optic nerve, ciliary vessels, and nerves pierce it from behind. It is thickest posteriorly, where it corresponds to the situation of the retina, and it becomes gradually thinner in front, until within a short distance of the cornea, when it again increases in thickness, the capsule of Tenon being here fused into its substance: it is thinnest immediately behind the insertion of the recti and obliqui muscles. The sclerotic is in relation externally with the capsule of Tenon, and internally, in front, with the ciliary muscle, and behind with the choroid. The Optic nerve passes through the sclerotic, together with the retinal vessels, at a spot about one-tenth of an inch internal to the antero-posterior axis of the eye. "Atlas of Surgical and Topographical Anatomy," by B. F. Beraud; translated by R. H. Holme, pl. 15, fig. 2. The passage through which the nerve enters the eye is funnel-shaped-being smaller towards the inner than the outer surface of the sclerotic: this opening is crossed by numerous decussating fibrous bands, which constitute the lamina cribrosa-in fact, it would be more correct to say that the sclerotic is pierced by a number of small openings for the transmission of the component fascicles of the nerve, rather than by a single one for the nerve itself. sheath. The optic nerve is encased in a dense fibrous sheath, Disposition a portion of which, on reaching the sclerotic, becomes of its fused into its structure, strengthening it posteriorly. In addition to this, the neurilemma of the various bundles of which the nerve is composed is not prolonged into the eye, but quitting the nervous elements, which are further deprived of their white substance, it terminates in the fibrous meshes of the lamina cribrosa and anterior layers of the sclerotic. Donders* has described the sheath of the nerve as Donders' double, the two parts having a somewhat different des- views. tination. The larger, external portion, leaves the nerve as it is about to enter the eye, and passing outwards, becomes incorporated with the sclerotic, to which it contributes an additional outer layer at this part. The inner, more delicate portion, follows the nerve as far as the lamina cribrosa, which it helps to form, and then bends outwards to join the sclerotic towards its inner surface. The two portions of the sheath, in the normal condition of the parts, are closely united by a thin intervening layer of loose connective tissue; but in those who are predisposed to the affection known as staphy- Relation to loma posticum, the same observer describes the outer sheath as diverging prematurely from the inner one, and leaving a considerable interval between them, which in section appears triangular, and is occupied by an increased growth of connective tissue. In this condition of the parts, the sclerotic immediately around the optic disc is represented by the thin layer of the inner sheath, deprived of the support it usually receives from behind, and therefore prone to yield to intra-ocular pressure and give rise to staphyloma (see fig. 33). "Accommodation and Refraction of the Eye," by Donders, p. 378 (New Sydenham Society). Post. sta phyloma. THE CON- Palpebral portion. Papillæ Ocular portion. Its vascular sup ply. The Conjunctiva is essentially a mucous membrane, composed of an external stratum of epithelial cells resting on a basement membrane, beneath which the capillary vessels are situated. It lines the eyelids, and is continued over the anterior part of the eyeball; in the former situation it is known as the tarsal or palpebral conjunctiva, and in the latter as the orbital, or ocular conjunctiva. At its line of reflection from the lids to the eyeball, the membrane forms a loose fold, called the tarso-orbital fold; at the inner angle of the eye is a vertical fold, the plica semilunaris. The palpebral conjunctiva is extremely vascular and thick, and its free surface is elevated into numerous papillæ, each of which encloses one or more fine capillary loops, and a terminal nervous apparatus, the whole being encased in connective tissue. Beneath the basement membrane is a loose connective tissue, in which a number of solitary glands resembling those of the intestines are imbedded; and besides these, there are a row of some eighteen or twenty conglomerate glands, opening by as many ducts on the free surface of the tarso-orbital fold of the conjunctiva; they pour out an abundant watery secretion, which helps to lubricate the eye. The ocular conjunctiva is void of papillæ, and is bound down to the capsule of Tenon by connective tissue; anteriorly it is united with the sclerotic. It is supplied with a superficial and deep set of vessels, the former being derived from branches of the palpebral and lachrymal arteries, and the latter from the muscular and ciliary; these anastomose with one another, forming a zone of vessels round the circumference of the cornea, and from this circle small branches pierce the sclerotic and anastomose with the vessels of the iris and choroid. In consequence of this arrangement, when the latter structure is congested, the zone of rotic zone." vessels round the cornea becomes turgid also, forming the "sclerotic zone of vessels," the "arthritic ring" of which we shall have to speak so frequently, as a most important indication of disorder in the intra-ocular circulation. The "scle Venous anastomoses. The veins of the conjunctiva empty themselves into the cavernous sinus through the muscular and lachrymal veins, and also into the angular vein of the face, by the nasal arch; so that if from any cause the pas sage of blood through the vasa vorticosa of the choroid into the ophthalmic vein is impeded, as in glaucoma, a collateral circulation is established through the veins of the conjunctiva—hence the enlarged and tortuous superficial vessels noticed in chronic diseases affecting the choroid. The Cornea is a modification of the sclerotic so con- THE CORstructed as to receive its nutriment by endosmosis, thus NEA. preventing the necessity for a vascular system, which would of course interfere with its transparency. The circumference is bevelled in such a manner that the sclerotic overlaps it; but with this exception, it is of the same thickness throughout. The cornea is divided into three lamina; the external Its anterior or conjunctival is an apparently structureless mem- lamina. brane, its anterior surface being covered by several layers of epithelial cells; posteriorly it sends processes inwards, interlacing with the fibrous elements of the lamina beneath it. The middle lamina constitutes the Middle principal bulk of the cornea, and consists of fibrous lamina. tissue, so arranged as to form strata superimposed one over the other; frequent communications, however, exist between contiguous layers, so that they are intimately connected one with another. In the intervals between the bundles and layers are innumerable interspaces or fissures, many of which contain an elongated nucleus; these spaces are probably filled with nutrient fluid during life. Branches of the long ciliary nerves may be traced into the cornea, where they appear to form a very abundant and intricate network. The internal lamina of the cornea is composed of an Internal homogeneous membrane, and is lined internally-that lamina. is, towards the aqueous humour, by epithelial cells. Bowman describes it as "a transparent homogeneous membrane. Though very hard and capable of resisting pressure, giving a crisp sound when divided with scissors, yet it is very brittle and easily torn, fragments showing a remarkable tendency to curl up on all sides into rolls."* relations of A part of the fibrous structure of the middle lamina Anatomical unites with the internal at the circumference of the internal cornea, and their union gives rise to three sets of lamina. "Lectures on the Parts concerned in the Operations on the Eye," by W. Bowman, p. 19. |
