Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
45
Ocular
muscles.
Special sists of 98*10 parts in the 100 of water, a trace of albumen,
Anatomy, and about 2 parts of muriates and lactates of soda. It is
contained in a membrane, which lines the posterior sur¬
face of the cornea, and is supposed to cover the anterior
surface of the iris. This, however, is questionable. In
1768 a membrane of this kind was described by Demours
and Descemet, both of whom claimed the discovery with
great eagerness and some animosity. These rival anato¬
mists appear to have forgotten that the aqueous humour
may be secreted as well by the cornea and iris as by a
proper membrane.
The relation of the coats and humours of the eye to
each other may be understood by the diagram (fig. 3,
Plate XXX.), where A is the anterior chamber, P the pos¬
terior, L the lens, and c, c the ciliary processes. The
other parts are easily understood from the foregoing de¬
scription.
The eye is supplied with blood chiefly by the ophthalmic
artery.
The eyeball, thus constituted, is moved in different
directions by six muscles, is moistened externally by fluid
secreted from a particular gland, and is protected from ex¬
ternal bodies by the eyelids and their appendages.
Of the muscles, four are straight and two oblique. The
former (attollens, depressor, adductor, abductor'), attached to
the margin of the optic hole, and terminating in tendons
inserted into the superior, inferior, nasal, and temporal
parts of the eyeball, raise, depress, adduct and abduct
the organ. Of the two latter, the superior oblique (troch-
ledris), which is attached to the margin of the same hole,
passes through a pulley-like cartilage at the inner margin of
the vault of the orbit,and is inserted into the internal region
of the ball, rolls the eye forward and inward, and turns the
pupil outwards and downwards ; while the inferior oblique,
attached to the orbitar process of the superior maxillary
bone, and inserted between the adductor and the optic
nerve, rolls it forwards and turns the pupil upwards. These
muscles, which occupy the apex of the orbit, are sur¬
rounded by a thick cushion of fat, on which the eyeball
rolls in its movements. (Plate XXXIII. fig. 6.)
In the hollow, at the outer temporal region of the or¬
bitar vault, is placed the lacrymal gland, a granular gray¬
ish body, about the size of a bean, consisting of lobules,
with arteries in the intermediate furrows, derived from a
branch of the ophthalmic, and accompanying veins. (Plate
XXXIII. fig. 5 and 7, G, g.) These constituent lobules
have been represented, on the authority of Steno in the
ox, and the elder Monro in the human subject, to ter¬
minate in 7 or 8 minute excretory ducts, opening on the
conjunctiva. In man, however, they were sought in vain
by Duverney, Morgagni, Haller, and Zinn; and neither
Portal nor Bichat have been able to satisfy themselves of
the existence of these ducts. It is nevertheless certain
that the lacrymal gland secretes the tears, and that the
latter issue from its lobules.
The eyes are covered anteriorly by two musculo-mem-
branous folds named the eyelids (palpebrce), attached to
the margins of the orbit, and forming by their free mar¬
gin the palpebral opening, with commissures at each angle
(canthus nasalis, et canthus temporalis').
The upper eyelid, which is large, is bounded above by
the eyebrow {supercilivm), a cutaneous eminence, arched
transversely, covered with hairs, and with the corrugator
supercilii attached to its nasal end. Between each eye¬
brow is a smooth space, named the glabella or mesophryon.
Into the upper eyelid the levator (fig. 5, L) is inserted.
Each eyelid consists of skin externally, mucous mem¬
brane within {conjunctiva), intermediate cellular tissue,
muscle, and a fibrous membrane, attached by one margin
to the base of the orbit, and terminating by the other in
Lacrymal
gland.
Eyelids.
the tarsal fibro-cartilages. These are crescentic bodies Special
placed in the free margin of the eyelids, and by their Anatomy,
firmness and elasticity giving the requisite tension to the
eyelids when the orbicular muscle acts, or the levator is'1'^8! and
relaxed. The cutaneous border of the tarsi is occupied^ an
by a range of short, firm hairs, named the eyelashes {ci¬
lia). In the mucous borders are the orifices of the tarsal
or Meibomian follicles, of the same character as the mu¬
ciparous follicles generally. These are placed in the sub¬
stance of the eyelid, beneath the conjunctiva, and behind
the tarsal fibro-cartilages. From the inner surface of the
eyelids the palpebral conjunctiva is continued over the
anterior part of the sclerotic and cornea, forming the oph¬
thalmic conjunctiva.
In the nasal angle is lodged a minute red body, named Caruncle,
the caruncle {caruncula lacrymalis), chiefly consisting of
filamentous tissue and vessels covered by mucous mem¬
brane ; and behind this is a fold of the membrane, which
has been named membrana nictitans, large in the lower
animals, but often so imperfect in man as to be merely
rudimentary.
At the nasal end of each eyelid is a minute capillary Lacrymal
orifice which leads into a horizontal canal, terminating in orifices,
a membranous sac lodged in the depression of the lacrymal
bone. These orifices, which are named the puncta lacry-
malia {p,p, fig. 5), are the superior or palpebral openings of
the lacrymal sac and passages, the lower aperture of which
is found in the inferior nasal meatus. The tears effused
from the lacrymal gland at the temporal region of the orbit
are carried, by the frequent action of the orbicular muscle,
over the ball, till they reach the nasal angle, where they are
gradually absorbed by the capillary orifices of the. puncta,
and conveyed into the sac, and eventually to the nose.
The eye derives nerves from six different sources, all Ocular
of which, however, maybe distinguished into three classes,nerves*
the sensitive, motive, and entrophic nerves. The first
consists of the second, optic or the proper visual nerves.
The second class comprehends the third, fourth, and sixth
nerves, of which the third or oculo-muscular are distributed
to the levator palpebrce superioris, the attollens, the adduc¬
tor, the depressor, and the obliquus inferior; the fourth
is entirely distributed to the obliquus superior; while the
sixth pair are confined to the abductor. The third class
of nerves is derived from the ophthalmic or quadrilateral
ganglion, which is formed chiefly from the junction of a
sub-branch of the naso-ocular branch of the first or oph¬
thalmic division of the fifth pair, with a small branch of
the third nerve. From this arise a small superior cluster
of three nerves adhering to the optic, and a large inferior
cluster of eight or ten nerves, which quickly join the
ciliary arteries, and are with them distributed in the
ciliary circle and posterior part of the iris. (Plate XXXHI.
fig. 7.) From the other branch of the naso-ocular nerve,
the caruncle and lacrymal canal, with the orbicular muscle
and epicranius on the one hand, and the lacrymal gland
on the other, receive nervous filaments.
SECT. III.—THE ORGAN OF HEARING; THE EAR.
The organ of hearing consists of the auricle or external
ear, with the ear-hole; the middle ear, including the tym¬
panal cavity and its appendages; and the internal ear or
labyrinth.
The auricle is a fibre-cartilaginous substance, moulded Auricle,
into a conchoidal shape, covered by skin, attached to the
cranium by ligaments, and susceptible of motion by
muscles. It is common to distinguish in it the following
parts. The helix, a semicircular eminence above the ear-
hole ; the groove of the helix below it; the antihelix, an
eminence commencing in the groove by a superior, broad,