dius of curvature of the cornea is less than that of the sclera, giving the effect of a crystal placed on the face of a watch. Owing to the projection of the cornea, the antero-posterior diameter of the eye is the longest-approximately 0.95 inch (24.2 mm.); the transverse diameter is 0.925 inch (23.6 mm.); the vertical, 0.915 inch (23.2 mm.), and the oblique 0.943 inch (24 mm.). The Orbital Cavities-(Orbitae) The orbits are two cavities, conoidal in shape, situated on either side of the nose, and varying slightly in size and shape, according to race, age, and sex. The cavities extend backward toward each other an angle of about 45 degrees, terminating at the optic foramen, which forms the apex of the cone-like cavity opposite the sella turcica of the sphenoid bone. The bases, or external openings are directed outward and forward. From before backward each cavity measurs approximately 134 inches (40 mm.). From side to side, near the external openings, they measure 11⁄2 inches (35 mm.) and from above downward, in the same situation, 14 inches (30 mm.), all dimensions being slightly less in the female. Both orbits together are formed of eleven bones, of which three, the ethmoid (os estmoidale), the frontal (os frontale), and the sphenoid (os sphenoidale), form part of the upper, inner and outer walls of both orbits. The remainder, namely the lacrimal (os lacrimale), the malar, or zygomatic (os zygomatica), the palate (os palatina), and superior maxillary (or maxilla) are paired, or one for each orbit. For accurate descriptive purposes it is convenient to divide each cavity into a floor, a roof, two walls, a base, a circumference, an apex, and four angles. The floor of the orbit is formed by the orbital surface of the surface of the superior maxillary part of the malar, or zygomatic, and the palate bones. The surface is relatively flat, and presents the palato-maxillary suture and infraorbital groove in back. Anteriorally, on the floor, is found the infraorbital canal. The roof of the orbit is formed by the frontal and lesser wing of the sphenoid bone. Its surface. is concave, the lacrimal gland being situated at the outer angle in the lacrimal fossa (fossa glandular lacrimalis). At the inner angle is the fovea trochelaris, a small, shallow depression for the pulley of the superior oblique muscle. The inner, or medial wall of the orbit is formed by the nasal process of the maxillary, the lacrimal, ethmoid, and a small part of the body of the sphenoid bone. It presents a groove for the lacrimal sac (fossa sacci lacrimalis), the lacri mal crest (christa lacrimalis), both anteriorally, and two sutures, the ethmo-lacrimal and the ethmo-sphenoidal. The external, or lateral wall, which is the densest in structure, is formed by the malar, or zygomatic, and the greater wing of the sphenoid bone. The base of the orbit, which corresponds to the external opening, is formed by the frontal malar (or zygomatic), and superior maxillary bones. It will be noticed that the margins project forward and inward to a slight extent, so that the orbital circumfrence, or rim, is somewhat less in diameter than the bordering portion of the cavity. It presents the supraorbital margin, formed from the frontal bone, and the infraorbital margin, formed by the superior maxillary and a small portion of the malar (or zygomatic). The apex of the orbit is at the inner and upper part of the body and lesser wing of the spehnoid and part of the ethmoid, where the optic foramen is situated. The angles are: The superior external, which is the point of junction of the frontal with the malar (or zygomatic) bone; the superior internal, formed by the junction of the frontal with the lacrimal in front and the ethmoid bone behind; the inferior external, formed by the junction of the greater wing of the sphenoid, malar (or zygomatic) and palate bones; the inferior internal, resulting from the union of the lacrimal, ethmoid, superior maxillary, and palate bones. The orbits are covered by the periorbita, which is a membrane-like structure, called periosteum. This surruonds all the bones except over their articular surfaces. It consists of a moderately dense layer of fibrillar connective tissue, closely applied to the bone and having numerous blood vessels, which enter the bone and supply it with nourishment. Periosteum also contains elastic fibers, and a system of lymphatic vessels as well a nerve which, as in the case of the blood vessels, pass from it into the bone tissue. In incompletely grown bone, the inner layer of the periosteum contains irregular layers of large cells, called osteoblasts, from which new bone tissue is formed. The periorbita has openings for the passage of the various vessels and nerves passing in and out of the orbital cavitay. It is continuous with the dura mater, or outer covering of the brain in the back at the optic foramen, being connected with the sheath of the optic nerve. In front it separates to form the capsule of the lacrimal gland. The periorbita is also continuous with the periosteum of the bone outside of the orbital cavity proper. The eyeball occupies the front and larger half of the orbit. It is situated almost in the center, but is somewhat nearer the upper and outer walls than it is to the other parts. The posterior hemisphere of the eye is in relation with the ocular muscles and intervening fat, being located midway between the external opening ond the optic foramen. The anterior one-third of the eyeball is practically free except for the covering of the conjunctiva. It projects beyond the opening of the orbit to a slight degree, the amount of projection varying with different individuals, according to the amount of orbital fat and the size of the eyes, as well as the shape of the orbit. A straight line drawn from the inner to the outer orbital margins will strike the eye behind the cornea somewhere near the orra serrata. The globe or bulb is held in position by beds of connective tissue and the orbital muscles, the space between which are occupied by the orbital fat, which exists in greatest amount immediately behind the eyeballs, then extending forward as far as the anterior half of the cavity. The optic nerve is also surrounded by the orbital fat as well as by a loose connective tissue. The structures within the orbit are held together by fibrous tissues, which are connected with each other and are termed the orbital fascia. Part of this fascia is derived from the periorbita and part from Tennon's capsule. There are nine openings in and around the orbits, variously termed as fissures (meaning a cleft or a groove), foramina (a hole or perforation, especially in bone), canals (any tubular and relatively narrow passage or channel), and ducts (a passage with well-defined walls-applied especially to a tube for the passage of secretions or excretions). Various structures pass through these openings, as follows: 1. The optic foramen, which transmits the optic nerve and central artery. 2. The sphenoidal, or superior orbital fissure, transmitting the third, fourth, sixth, and ophthalmic division of the fifth cranila nerves, and filaments of the sympathetic nerves, the ophthalmic veins, and branches of the lacrimal and middle meningeal arteries. 3. The spheno-maxillary fissure, transmitting the superior maxillary nerve, Meckel's ganglion, and the maxillary artery. 4. The supraorbital foramen, transmitting the supraorbital nerve, artery and vein. 5. The infraorbital foramen, transmitting the infraorbital nerve and artery. 6 and 7. The anterior and posterior ethmoidal foramina, transmitting arteries, nerves and veins of the same name. 8. The malar foramen (foramina zygomatico 6. 7. 8. The bulb, or globe. Lacrimal glands and ducts. Sclera (Tunica Fibrosa Oculi) The sclera, so named on account of its firm consistence, is a part of the external or outer tunic of the eye. It is a semi-transparent whitish membrane made up of fibrous, connective and elastic tissue which interlace and form a latticelike network, in the interspaces of which are found blood vessels, nerves, lymph, pigment and stellate-formed cells. The sclera forms the posterior 5/6 of the ocular capsule and extends from the sheath of the optic nerve with which it blends posteriorly, to the cornea, anteriorly where it is covered by the conjunctiva. It has two surfaces, external and internal; the internal surface is attached (most firmly in front) to the choroid by means of an irregular meshwork of pigmented cells termed the lamina fusca. The external surface gives attachment to the ocular muscles just in front of the equatorial line surrounding the sclera and the various structures which pass in and out of it, is the capsule of Tennon (posterior twothirds) and the conjunctiva (anterior one-third). The sclera is thicker in back 1.25th of an inch (1 millimeter) to 1.60th of an inch (0.5 millimeter) in front. The sclera has numerous openings for the passage of blood vessels and nerves. Behind and about 1.6 of an inch (4 millimeters) to the nasal side just below the level of the posterior pole is a perforated or sieve-like portion, the lamina cribrosa sclerae which transmits the nerve bundles of the optic nerve which enters the eyeball at this point. This lumina also has a comparatively large central opening called the porous opticus through which the central artery of the retina passes. Around the porous opticus are a series of small openings about 20 in number for the passage of the ciliary nerves and the short |