The globe or ball of the eye is a composite structure of spheroidal form, placed in the fore part of the orbital cavity, and receiving the thick stem of the optic nerve behind. The recti and oblique muscles closely surround the greater part of the eyeball, and are capable of changing its position within certain limits: the lids, with the plica semilunaris and caruncle, are in contact with its covering of conjunctiva in front; and behind it is supported by a quantity of loose fat and connective tissue.
The eyeball is composed of segments of two spheres, of which the anterior is the smaller and more prominent: the segment of the larger posterior opaque sphere corresponds with the limit of the sclerotic coat, and the translucent portion of the smaller sphere with that of the cornea.
From before backwards the ball measures about nine-tenths of an inch, and its transverse diameter exceeds this measurement by about a line.
Except when directed towards near objects, the axes of the eyes arc nearly parallel ; the optic nerves, on the contrary, diverge considerably from one another, and each nerve enters the corresponding eye about a tenth of an inch to the inner or nasal side of the axis of the globe.
The eyeball consists of several concentric coats, and of certain fluid and solid parts contained within them. The coats or membranes are three in number, viz.: an external fibrous covering, named sclerotic and cornea; a middle vascular, pigmented, and in part also muscular membrane, the choroid and the iris: and an internal nervous stratum, the retina. The enclosed refracting media, three in number, are the aqueous humour, the vitreous body, and the lens with its capsule.
Around the eyeball there is an adventitious tunic of fascia, tunica vaginalis oculi, or capsule of Tenon, which is perforated by the tendons of the recti and obliqui muscles, and connected with the sclerotic by merely the most delicate connective tissue. This capsule, which in reality consists of two membranous layers lined by flattened epithelioid cells, and enclosing a lymph space, separates the eyeball from the orbital fat, and enables it to glide freely in its movements.