_1_, Gland; _2_, fat surrounding deep part of cartilage. (After
Ellenberger, in Leisering’s Atlas.)
]
The =conjunctiva= is the mucous membrane which lines the lids as
=palpebral conjunctiva= (C. palpebrarum) and is reflected upon the
anterior part of the eyeball as =bulbar conjunctiva= (C. bulbi); the
line of reflection is termed the =fornix conjunctivæ=. The palpebral
part is closely adherent to the tarsus, but is loosely attached further
back. It is papillated and is covered with stratified cylindrical
epithelium in which many goblet cells are present. In the fornix and its
vicinity there are tubular glands (of Henle). Near the inner angle there
are numerous lymph nodules. The conjunctiva of the outer part of the
upper lid is pierced near the fornix by the orifices of the excretory
ducts of the lacrimal gland. The bulbar conjunctiva is loosely attached
to the anterior part of the sclera and is pigmented in the vicinity of
the corneo-scleral junction. On the cornea it is represented by a
stratified epithelium.
When the lids are in apposition, the conjunctiva (including the
epithelium of the cornea) forms a complete =conjunctival sac=, as the
capillary space between the lids and the eyeball is termed.
The =third eyelid= (Palpebra tertia) is situated at the inner angle of
the eye (Figs. 551, 556). It consists of a semilunar fold of the
conjunctiva, known as the =membrana nictitans=, which covers and partly
incloses a curved plate of hyaline cartilage. Its marginal part is thin
and usually more or less pigmented. The cartilage has an elongated
quadrilateral outline. The part of it which lies in the membrana is wide
and thin. The deep part is narrower and thicker and is embedded in fat
at the inner side of the eyeball. Numerous minute lymph nodules occur in
the membrana nictitans, and the deep part of the cartilage is surrounded
by a gland which resembles the lacrimal gland in structure (Glandula
superficialis palpebæ tertiæ).
Ordinarily the third eyelid extends very little over the inner end of
the cornea, but when the eyeball is strongly retracted (as in tetanus)
the membrana is protruded over it so as to measure about an inch (ca.
2 to 3 cm.) in its middle. This effect results from the pressure of
the eyeball and its muscles on the fat which surrounds the deep part
of the cartilage.
=Vessels and Nerves.=—The =arteries= which chiefly supply the eyelids
and conjunctiva are branches of the ophthalmic and facial arteries, and
the blood is drained away by corresponding veins. The =sensory nerves=
are branches of the ophthalmic and maxillary divisions of the
trigeminus. The =motor nerves= to the orbicularis oculi, corrugator
supercilii, and malaris come from the facial nerve, the levator palpebræ
superioris is innervated by the oculomotor, and the unstriped muscle of
the lids by the sympathetic.