The =eyeball= (Bulbus oculi) is situated in the anterior part of the
orbital cavity, protected in front by the eyelids and conjunctiva, in
its middle by the complete orbital ring, and related behind to the
bulbar fascia, fat, and ocular muscles.
It has the form approximately of an oblate spheroid, but is composed of
the segments of two spheres of different sizes. The anterior,
transparent, corneal segment has a radius of curvature of about 17 mm.,
and the posterior, opaque, scleral segment a radius of about 25 mm. The
anterior segment therefore projects more strongly, and the junction of
the two segments is marked externally by a broad, shallow groove, the
sulcus scleræ. The central points of the anterior and posterior
curvatures of the eyeball are termed respectively the =anterior= and
=posterior poles= (Polus anterior, posterior), and the line connecting
the poles is the =optic axis= (Axis oculi externa). The angle of
divergence of the optic axes is about 137 degrees. The =equator=
(Æquator) is an imaginary line drawn around the eyeball midway between
its poles, and =meridians= (Meridiani) are lines drawn around it through
the poles.
The average transverse diameter of the eyeball is about 5 cm., the
vertical about 4.5 cm., and the axial about 4.25 cm. The distance from
the anterior pole to the point of entrance of the optic nerve is about
3 cm.
[Illustration:
FIG. 556.—LEFT EYEBALL OF HORSE, _in situ_, AFTER REMOVAL OF UPPER AND
LOWER LIDS.
_9_, Zygomatic arch; _12_, supraorbital process; _19_, orbital fat;
_27_, facial crest. (After Ellenberger-Baum, Anat. für Künstler.)
]
The eyeball consists of three concentric tunics or coats, within which
three refractive media are inclosed.