Page:The American Cyclopædia (1879) Volume VII.djvu/54

 EYE orbits, the axes of the eyes are parallel. In front, the globe of the eye is in relation with the reflection of the mucous membrane of the lids; behind and all around, with the muscles, vessels, nerves, and a cushion of soft fat. The eye is com- posed of membranes and humors. Of the mem- branes of the eye, the cornea has already been described under its own title; the others are the sclerotic, choroid, ciliary processes, iris, and retina. The sclerotic is the external mem- brane, forming the posterior four fifths, the an- terior fifth being formed by the cornea; it is white, firm, and resisting, opaque, thick, and composed of interlaced fibres. Beneath the sclerotic is the choroid, composed of small ar- teries and veins united by delicate areolar tis- - sue; it extends from the entrance of the optic nerve forward to the ciliary circle; both its surfaces are covered with a dark pigment, which gives the deep color seen in the interior of the eye. The ciliary circle or ligament is a grayish ring, a line or two wide, united by its larger circumference to the choroid, and by its lesser to the iris; the ciliary processes are membra- nous folds, 60 to 80 in number, extending from the choroid to the neighborhood of the opening of the pupil; they form by their union a ring behind the iris and in front of the vitreous hu- mor, surrounding the crystalline lens like a crown. At a short distance behind the cornea is the circular, vertical, membranous curtain, the iris, pierced in the middle by the pupil; this curtain hangs in the aqueous humor, sepa- rating it into the anterior and posterior cham- bers of the eye; it presents anteriorly a great number of radiations converging toward the pupil, the muscular fibres for the dilatation of this opening, and is variously colored in differ- ent individuals; the posterior surface has a number of circular fibres for contracting the pupil, and is covered with a thick dark pig- ment layer called uvea; both surfaces are lined with the delicate membrane of the aqueous hu- mor; the greater circumference is connected with the ciliary ligament and processes; its movements are doubtless partly owing to its erectile and vascular tissue. Beneath the cho- roid is the retina, a thin soft expansion of the optic nerve, surrounding the vitreous humor and extending forward as far as the ciliary pro- cesses and crystalline lens; about two lines to the outside of the tubercle of the nerve it pre- sents a circular dark spot and a small perfora- tion discovered by Sommering. The retina is the immediate organ of vision, which receives the rays of light and transmits the visual im- pressions by the optic nerve to the sensorium. Of the humors of the eye, the crystalline lens has been described under that head; the others are the aqueous and vitreous humors. The aqueous humor is a limpid transparent fluid, varying in quantity from four to six grains, oc- cupying the space in front of the lens which is divided into anterior and posterior chambers by the iris; it contains in solution a little albu- men and the salts usually found in such secre- tions; when lost by accident or in the opera- tion for cataract by extraction, it is speedily formed again. The vitreous humor occupies the posterior three fourths of the globe of the eye, having the lens encased in its anterior portion; it consists of a transparent, gelatinous fluid enclosed in a great number of cells formed by the partitions of the hyaloid membrane, communicating with each other; in the ope- ration for cataract by depression the lens is pushed backward and downward into this hu- mor. The optic nerves are the second pair of cerebral nerves. The globe of the eye is moved by six muscles arising from the contour of the optic foramen and its vicinity, and attached to the sclerotic coat; of these muscles four are straight, called the external, internal, superior, and inferior recti muscles, moving the eye re- spectively outward, inward, upward, and down- FIG. 2. Muscles of the Eyeball viewed from above and from the inner side. S. JR., superior rectus; Inf. fi., inferior rectus; J. JR., exter- nal rectus; In. R., internal rectus; S. ob., superior oblique; Inf. ob., inferior oblique; Ch., cbiasma of the optic nerves (77.); 777., the third nerve, which supplies all the muscles except the superior oblique and the external rectus. ward. The first two muscles are often perma- nently contracted, producing divergent or con- vergent strabismus, a deformity curable by the division of the contracted muscles, a simple and comparatively painless and bloodless op- eration; the superior oblique muscle passes through a pulley in the inner portion of the orbital process of the frontal bone, from which it extends to the posterior and external part of the globe, rotating the organ inward and forward; the inferior oblique passes from the internal and anterior part of the floor of the orbit to the external and posterior surface of the globe, rotating the eye outward and up- ward. The conjunctiva, the mucous mem- brane of the eye, is reflected from the lids and covers the anterior portion of the globe; it is in this membrane that the redness and swelling of ordinary ophthalmia have their seat. The eye is frequently destroyed by accident or disease; in cases of removal of the organ artificial eyes are used to remedy the deformity; these are made of glass and enamel, and when having the natural size, shape, coloration of iris, form of pupil, projection of cornea, tint of sclerotic, and vascularity, it is often very difficult to de tect the real from the artificial organ, especially 'i