Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
43
Special brane mucous glands are indistinct; but we recognise mi-
Anatonw. nute orifices or pores, which may be the follicular cavities
on a small scale, or the orifices into which the mucous
fluid is poured after secretion by the arteries.
This mucous membrane is supplied from the internal
maxillary artery with blood-vessels, which are at once
abundant and superficial. These anastomose with the in¬
fra-orbital and ethmoidal branches of the ophthalmic ar¬
tery and some others of the internal carotid. On these
circumstances depends the frequency of hemorrhages
from this membrane in early life, while the capillary sys¬
tem is energetically employed in the enlargement of the
cranio-facial region; and in advanced life, when venous
plethora is most conspicuously evident in the vessels of
the head.
The nerves which supply the nasal cavities are the first
or olfacient nerves principally, the internal nasal of the
ophthalmic, and several branches derived from the spheno¬
palatine ganglion, the frontal, the great palatine, and the
vidian. The distribution of these nerves has been beau¬
tifully represented by Scarpa (Annot. Acad.), from whom
fig. 1 and 2 of Plate XXXIII. are imitated.
SECT. II. THE ORGAN OF VISION ; THE EYES.
The eyes, placed in the orbits, are distinguished into the
globes of the eyes, and their appendages.
The globe or ball of the eye, situate at the base of the
orbit, has a spheroidal shape, with the antero-posterior
diameter longest, and varying in the adult from 10 to 11
lines. The direction of the eye differs from that of the
orbit, the axis of which is oblique and convergent behind,
while that of the two eyes is parallel.
The eye-ball (bulbus oculi) consists of several mem¬
branes, containing fluid or semifluid substances, denomi¬
nated humours. The external and strongest is the sclerotic,
with the cornea enchased in its anterior aperture ; next is
the choroid and retina; and the iris, a circular membrane,
with an annular aperture, is placed transversely across,
dividing the cavity into anterior and posterior chambers.
The humours are the vitreous, the crystalline lens, and the
aqueous humour.
The sole- The sclerotic, sometimes named cornea opaca, in contra-
rotic. distinction to the clear cornea, is shaped like a spherical
shell, truncated before, and is estimated to occupy about
four fifths of the globe. Its exterior surface is covered by
the adipose matter of the orbit behind, and by the tendons
of the muscles of the eye and the ophthalmic conjunctiva
before. Its inner concave surface is lined by the choroid
coat. In its anterior opening, which is about six or seven
lines diameter, the cornea is enchased by imbrication of
the sclerotic; and in a posterior opening about one and
a half line in diameter, the ophthalmic end of the optic
nerve is fixed. It is fibrous in structure, becoming trans¬
lucent when immersed in oil of turpentine. Its vessels are
generally colourless, and it appears void of nerves.
I he cor- The cornea, often named the clear or transparent cornea
(cornea lucida), to distinguish it from the opaque cornea or
sclerotic, which was supposed by the old anatomists to be
of the same nature, is a segment of a smaller sphere than
that of the sclerotic, attached to its anterior aperture, and
occupying the anterior fifth of the eyeball. Before it is
covered by a thin pellicle continued from the conjunctiva,
behind by the anterior part of the membrane of the aque¬
ous humour; and its circumference is inseparably enchas¬
ed within that of the anterior aperture of the sclerotic.
I he cornea consists of several concentric layers of transpa¬
rent homogeneous matter, void of vessels or nerves, closely
united, but separating and becoming opaque after death.
The choroid coat (tunica choroides) lines the inner sur- Special
face of the sclerotic, to which it adheres loosely, except at Anatomy,
the insertion of the optic nerve, where it has a posterior
opening, yet may be easily detached. The margin of its -f
antenoi opening, which is large, adheres to the ciliary cir¬
cle and processes, and appears to be continuous with the
ins.. The outei suiface is covered by a brownish-black
viscid matter, which partly adheres to the inner surface
of the sclerotic. The inner surface, over which the retina
is extended without adhesion, is covered by the same
brownish-black matter; but of this the retina receives none,
as the sclerotic. The choroid is a thin membrane, of a
grayish colour when deprived of its brown pigment, trans¬
lucent, homogeneous, and, so far as observation goes,
void of fibres, but liberally supplied by minute blood-ves¬
sels, partly from the long, chiefly from the posterior cilia-
ries, which, dividing on its outer surface into numerous
ramifications, mutually approximate, and anastomosing,
form a network of quadrilateral and trapezoidal meshes.
The tunica Ruyschiana is imaginary, unless this arrange¬
ment of capillary vessels be such.
From these vessels is derived the brownish-black mat- Pigm»n-
ter (pigmentum nigrum) with which the retinal surface of turn m-
the choroid is covered. Its appearance on the convex orSrum*
sclerotic surface is the effect of cadaveric transudation.
The nature of this colouring matter is imperfectly known.
When first secreted it is brown, but becomes black succes¬
sively as it continues. It is not affected by the action of
light or caloric, and undergoes no change from the opera¬
tion of the mineral acids, aqua potassce, ammonia, and al¬
cohol. It appears to consist chiefly of carbonaceous mat¬
ter. Menghini asserts that he has obtained from it minute
particles of iron attracted by the magnet.
The anterior orifice of the choroid is firmly connected Ciliary cir-
to a thick ring of grayish pulpy substance, and forming cle.
the point at which the sclerotic and cornea without, and
the iris within, are united. This ring, named the ciliary
circle (ligamentum ciliare), is readily detached from the
sclerotic. Its structure is unknown.
Posterior to this is a range of prominent minute bodies, Ciliary
with free extremities, lying over the crystalline lens, va-P1-0^856*5*
rying in number from seventy to eighty. These are the
ciliary processes (processus ciliares). They are trilateral-
prismatic in shape, about a line and a half long, more dis¬
tinct and longer in the human eye than in those of most
brute animals. Their intimate structure is not very well
known ; but they are highly vascular, and their vessels ap¬
pear to be capable of occasional erection. (Plate XXX.
fig- 4.)
Anterior to the ciliary circle is the iris, a circular mem-The iris,
brane, placed in the transverse vertical position, with an¬
terior and posterior surfaces, and a circular opening in
the centre. The exterior ciliary margin (annulus major)
is attached, as already stated, to the ciliary circle. The
inner or pupillar (anmdus minor) is free, and forms what
is named the pupil of the eye. The anterior surface is
marked by a variety of colours, blue, gray, or black,
brown, or that peculiar brownish-black named hazel, and
which are not unfrequently allied with the tints of the
skin and hair. The same surface presents radiating lines,
which pass from the small to the large circle diverging.
Both appearances are produced by vascular arrangement.
The posterior surface of the iris, which has been distin¬
guished as a separate membrane under the name of uvea,
is covered by the same dark-coloured matter secreted by
the choroid, generally more abundant, and of a deeper
tint, than the pigment of that membrane. When this is
removed by washing, the observer recognises the radiated
streaks impressed by the ciliary processes. (Fig. 4.)