The =eyelids=, upper and lower (Palpebra superior et inferior), are
movable folds of integument situated in front of the eyeball. When
closed, they cover the entrance to the orbit and the anterior surface of
the eyeball. The upper lid is much more extensive and more movable than
the lower one, and its free edge is more concave. The interval between
the lids is termed the =palpebral fissure= (Rima palpebrarum). When the
eye is closed, it is an oblique slit about two inches (ca. 5 cm.) in
length; when open, it is biconvex in outline. The ends of the fissure
are the =angles= or =canthi=, and are distinguished as =internal= or
=nasal=, and =external= or =temporal= (Angulus oculi medialis,
lateralis). The external angle is rounded when the eye is open, but the
internal angle is narrowed to form a ᑐ-shaped bay or recess, termed the
=lacrimal lake= (Lacus lacrimalis). In this there is a rounded,
pigmented prominence known as the =lacrimal caruncle= (Caruncula
lacrimalis); it is about the size of a small pea and is covered with
modified skin, connected with that of the inner commissure, from which
project a number of hairs provided with sebaceous glands. The lids unite
on either side and form the =commissures=, internal and external
(Commissura palpebrarum medialis, lateralis). The =anterior surface= of
the lids (Facies anterior palpebrarum) is convex and presents a variable
number of long tactile hairs in addition to the ordinary but very short
hairs. A considerable number of tactile hairs are scattered over the
lower part of the lower lid, but on the upper lid they are very scanty.
The infrapalpebral depression (Sulcus infrapalpebralis) indicates
somewhat indistinctly the limit of the lower lid. The upper lid is
marked by two furrows when raised. The =posterior surface= (Facies
posterior palpebrarum) is adapted to the free surface of the eyeball and
is covered by the palpebral conjunctiva. The free border of the lid is
smooth and usually black. It has a well defined posterior margin (Limbus
palpebralis posterior), along which the ducts of the tarsal glands open.
The anterior margin (Limbus palpebralis anterior) bears stiff hairs
termed the cilia or eyelashes. On the upper lid the cilia are long and
numerous except at its inner third, where they are very small or absent.
On the lower lid the cilia are often scarcely distinguishable from the
ordinary hairs; in other cases they may be clearly seen except near the
external canthus, and are much finer and shorter than those of the upper
lid. The edge of each lid is pierced near the internal angle by a
minute, slit-like opening, the =punctum lacrimalis=, which is the
entrance to the lacrimal duct or canaliculus.
[Illustration:
FIG. 552.—VERTICAL, SECTION OF ANTERIOR PART OF EYE OF HORSE, WITH
LIDS HALF CLOSED.
_1_, Tarsal gland of upper lid; _2_, palpebral conjunctiva; _3_,
fornix conjunctivæ; _4_, levator palpebræ superioris; _5_,
orbicularis oculi; _6_, cornea; _7_, anterior chamber; _8_, iris;
_9_, _9′_, granula iridis; _10_, posterior chamber; _11_, ciliary
process; _12_, ciliary muscle; _13_, ciliary zone or suspensory lig.
of lens; _14_, chorioid; _15_, sclera; _16_, lens; _17_, root of
tactile hair. (After Bayer, Augenheilkunde.)
]
=Structure of the Eyelids.=—The skin of the eyelids is thin and freely
movable, except near the free edge, where it is more firmly attached.
The underlying subcutaneous tissue is destitute of fat. The =muscular
layer= consists chiefly of the elliptical bundles of the orbicularis
oculi, with which are associated fibers of the corrugator supercilii in
the upper lid and the malaris in the lower lid. At the inner side there
is a fibrous band, the =palpebral ligament=, which is attached to the
lacrimal tubercle and furnishes origin to some fibers of the
orbicularis. At the inner commissure a bundle detached from the
orbicularis passes inward behind the lacrimal sac, and is known as the
pars lacrimalis or Horner’s muscle. At the outer side an indistinct
palpebral raphé occurs where fibers of the orbicularis decussate. The
=fibrous layer= is thicker and denser along the free edge of the lid,
forming here the =tarsus=. The tarsus furnishes insertion to a layer of
unstriped muscle known as the tarsal muscle (of Müller). The =tarsal
glands= (Glandulæ tarsales) are partly embedded in the deep face of the
tarsus, and are visible when the lid is everted if the conjunctiva is
not too strongly pigmented. They are arranged in a linear series, close
together, and with their long axes perpendicular to the free edge of
each lid. In the upper lid they number forty-five to fifty; in the
lower, thirty to thirty-five. Each consists of a tubular duct beset with
numerous alveoli, in which a fatty substance, the =palpebral sebum=, is
secreted. The =palpebral conjunctiva= lines the posterior surface of the
eyelids.
[Illustration: