1. =The Orbit.=--The bony framework of the orbital fossa, in which the
organ of sight is situated, has already been described (page 53). The
orbit is not protected by bone on all sides, being open caudad and
ventrad, and partly laterad. The structures within the orbit are further
protected and separated from other structures by a very thin, tough,
transparent membranous sac, the =periorbita=, which surrounds them
almost completely and nearly fills the orbit. The periorbita forms a
conical sac with the apex of the cone directed caudoventrad and attached
about the optic foramen and orbital fissure. The base of the cone is
attached about the margin of the orbital fossa,--to the supraorbital
margin of the frontal, to the maxillary and malar bones, and to the
orbital ligament, connecting the frontal process of the malar with the
zygomatic process of the frontal. The periorbita is partly separated
from the walls of the orbital fossa by masses of fat. It comes in
contact, aside from the bones, with the temporal and pterygoid muscles,
and with nerves and blood-vessels in the orbit.
The sac thus formed encloses the eyeball with its muscles and glands
(Fig. 166), as well as a mass of fat in which these are imbedded. The
periorbita and its contents are freely movable with relation to
surrounding structures.
2. =The Eyelids. Palpebræ.=--The eyelids are two thin folds which
protect the eye. Externally they are covered with hair; internally by
the thin membranous =conjunctiva=. The eyelids contain the =Meibomian=
or =tarsal= glands, arranged in short rows passing perpendicularly
inward from the edge of the lids; these may sometimes be seen with the
naked eye as broad yellowish lines. They secrete a substance which
prevents the adhesion of the edges of the two lids. On the edge of each
eyelid, three to four millimeters from the medial angle of the eye, is
one of the openings of the lachrymal canals.
At the medial angle of the eye is the large =nictitating membrane=
(=membrana nictitans=), or “third eyelid,” corresponding to the plica
semilunaris of man. In the cat this is large and may cover the whole
surface of the eye. It is supported (Fig. 165) by a broad central strip
of cartilage (_a_), passing from its edge to its inner angle, and the
inner end of this cartilage is surrounded, on the medial (concave) side
of the membrane, by the large lobulated =Harderian= glands (_b_).
[Illustration: FIG. 165.--INNER SURFACE OF MEMBRANA NICTITANS,
showing the supporting cartilage and Harderian glands. _a_, supporting
cartilage; _b_, Harderian glands; _c_, outer edge.]
The =conjunctiva= is the thin membrane covering the inner surface of the
lids, the outer surface of the eyeball, and both surfaces of the
nictitating membrane.
The muscles of the eyelids are =M. orbicularis oculi=, already described
(page 98), and =M. levator palpebræ superioris=, described below.
3. =Lachrymal Apparatus.=--The =lachrymal gland= is a large reddish
gland, not lobulated externally, which lies on the surface of the
eyeball just beneath the lateral angle of the eye. The gland is situated
immediately craniad of, and in contact with, the zygomatic process of
the frontal bone (Fig. 154, 11, page 374). It is flat and about one
centimeter in diameter. The ducts which pass from the gland to the eye
are not visible to the naked eye. The fluid secreted by the gland
collects at the medial angle of the eye and passes into the two openings
of the lachrymal canals, one of which is found on the pigmented edge of
each lid, three or four millimeters from the medial angle of the eye.
The canals passing from these openings soon unite to form the
=nasolachrymal duct=, which passes through the lachrymal bone, along the
medial surface of the maxillary, to open into the nasal cavity ventrad
of the ventral concha of the nose.
4. =Muscles of the Orbit= (Fig. 166).--Within the orbit are eleven
muscles, ten connected with the eyeball, and one, M. levator palpebræ
superioris, with the upper eyelid.
Of the ten muscles of the eyeball, eight are straight muscles, passing
from an origin about the inner end of the orbit directly distad; the
other two have an oblique direction. Four of the straight muscles are
larger and are known as the =Mm. recti= (_b_, _c_, _d_); these are
distinguished according to their position as =lateral= (_c_), =medial=,
=superior= (_d_), and =inferior= (_b_). The four smaller straight
muscles constitute together =M. retractor oculi= (_e_). The two oblique
muscles are known as =Mm. obliqui= and are distinguished according to
their position as =inferior= (_a_) and =superior=.
[Illustration: FIG. 166.--MUSCLES OF THE EYEBALL, LATERAL VIEW.
_a_, M. obliquus inferior; _b_, M. rectus inferior; _c_, M. rectus
lateralis; _d_, M. rectus superior; _e_, parts of M. retractor oculi;
_f_, the oculomotor nerve.]
=Mm. recti= (_b_, _c_, _d_).--The four recti muscles arise from the bone
about the optic foramen and pass toward the eyeball. They are inserted
by thin, flat tendons along a line which separates the darker caudal
part of the sclerotic from the white zone of the sclerotic which borders
the cornea,--the line of insertion of the four tendons forming thus a
circle about the eyeball. The rectus superior (_d_) is partly covered by
M. levator palpebræ superioris. The tendon of the rectus inferior (_b_)
is crossed by M. obliquus inferior (_a_). The recti muscles are all
innervated by the oculomotor (third) nerve (_f_), except the lateral
(_c_), which is supplied by the abducens (sixth) nerve.
=M. retractor oculi= (_e_).--The retractor oculi arises about the optic
foramen and divides into four heads which lie nearer the eyeball than
the recti and are therefore partly covered by the latter. They alternate
with the recti and are inserted into the eyeball at about its equator,
except the inferior division, which is inserted on a line with the
recti. They are supplied by the oculomotor nerve.
=M. obliquus inferior= (_a_).--The inferior oblique arises from the
maxillary bone just laterad of the lachrymal bone and curves over the
ventral side of the eyeball along the lines of insertion of the recti,
crossing the tendon of the rectus inferior (_b_) to the ventral edge of
the tendon of the lateral rectus (_c_), where it is inserted. It rotates
the eyeball and is supplied by the third nerve (_f_).
=M. obliquus superior.=--The superior oblique takes origin from the
cranial border of the optic foramen, passes dorsocraniad, and ends in a
small rounded tendon which passes through a fibrous pulley-ring. This
ring is situated near the rim of the orbit, one or two centimeters
craniad of the zygomatic process of the frontal bone, and is attached to
the wall of the orbit by two fibrous bands. After passing through the
pulley-ring the tendon turns and passes laterocaudad; it becomes much
expanded, passes caudad of the superior rectus, and is inserted into the
eyeball along the caudal margin of the insertion of the superior rectus
tendon. The superior oblique is supplied by the trochlear (fourth)
nerve.
=M. levator palpebræ superioris.=--This is a small thin muscle which
takes origin on the wall of the optic foramen close to the rectus
superior, passes over the outer surface of the rectus superior and
beneath the lachrymal gland, and forms a thin tendon which is inserted
into the margin of the upper eyelid. It is innervated by the oculomotor.
5. =The Eyeball.=--The eyeball is approximately spherical in form, but
the cranial surface has a slightly greater curvature than the caudal
portion, so that the eye appears slightly pointed at the cranial (free)
surface. The large cylindrical =optic nerve= (Fig. 167, _a_) enters the
eyeball at nearly the centre of the caudal half of the eye. On the outer
surface is seen the transparent =cornea= (_c_), covering the free
surface of the eye, and the opaque =sclerotic= (_b_), covering the
remainder of the ball. Through the cornea can be seen the yellow
curtain-like =iris=, with an opening in its centre, the =pupil=.
[Illustration: FIG. 167.--DIAGRAM OF A SECTION OF THE EYE.
_a_, optic nerve; _b_, sclerotic (_b′_, thickened “white zone”); _c_,
cornea; _d_, choroid; _e_, iris; _f_, ciliary body; _g_, retina; _h_,
crystalline lens; _i_, zonula ciliaris; _j_, anterior chamber of eye;
_k_, vitreous humor.]
=Coats of the Eye= (Fig. 167).--The outer coat of the eyeball is formed
by the =sclerotic= (or sclera) (_b_) and the =cornea= (_c_). The
sclerotic is the whitish opaque portion covering the caudal
three-fourths of the eye. It forms a tough sac serving as a protection
for the structures within. The cornea (_c_) is the circular transparent
portion of the outer coat, covering the free surface of the eye; it is a
direct continuation of the sclerotic. Just before passing into the
cornea the sclerotic is much thickened, forming a broad white zone
(_b′_) about the eyeball near its equator. To this zone, at its caudal
margin, are attached the rectus muscles of the eyeball. The sclerotic
has many pigment-cells on its inner surface, giving it a brownish
appearance and forming the =lamina fusca=.
The =cornea= (_c_) is transparent and more strongly curved than the
sclerotic. It is thicker than most of the sclerotic coat, but not so
thick as the white zone of the latter.
Within the sclerotic is the second coat of the eye, formed by the
=choroid= (_d_) and the =iris= (_e_). The choroid (_d_) is a vascular
layer, and contains much pigment, giving it a dark color. It lines the
inner surface of the sclerotic, but does not cover the inner surface of
the cornea. At the white zone of the sclerotic it is thrown into a large
number (about seventy) of meridional folds, the plicæ ciliares or
ciliary folds, which together constitute the so-called =ciliary body=
(=corpus ciliare=) (_f_). This forms a circular zone on the inner
surface of the white zone of the sclerotic, and is continuous with the
=iris= (_e_). The choroid coat is furnished in certain parts of its area
with a layer of cells containing crystals, which give it a brilliant
color, forming the =tapetum=. The color of the tapetum is a metallic
yellowish blue or green; it is this layer which causes the cat’s eye to
“shine” in the dark.
The =iris= (_e_) is a continuation of the choroid. It projects inward
from the white zone of the sclerotic, forming a circular curtain lying
some distance behind the cornea, and perforated by an opening, the
=pupil=. The iris is usually yellow in color on its outer surface,
darkly pigmented within. The pupil varies normally in size and form
according to the amount of light to which the eye is subjected. In a cat
killed with chloroform the pupil is very large and circular; in the
living animal it is elliptical with the long axis dorsoventral, or when
much contracted it is a mere dorsoventral slit.
The inner coat of the eye is formed by the =retina= (_g_). This is the
part of the eye which is sensitive to light; it is formed by an
expansion of the optic nerve (_a_). In a preserved eye it is usually
soft and opaque, and may be seen to line the caudal half of the inner
surface, extending apparently to the ciliary body. Here it seems to end
as a free margin, the =ora serrata=; it really becomes thin and passes
onto the surface of the ciliary body, forming the =ciliary portion= of
the retina, and onto the inner surface of the iris, where it forms the
uvea. Near the centre of the retina the entrance of the optic nerve is
marked by a small round spot, the =blind spot=.
The =Crystalline Lens= (_h_).--The lens is a biconvex transparent body
situated within the eye a little in front of the ciliary body and just
behind the iris. It is more strongly convex in front than behind. The
lens is surrounded by a thin transparent sac, the =capsule= of the lens
(capsula lentis). The capsule of the lens is attached all around the
equator of the lens by fibrous bands to the ciliary body, this
attachment forming the =zonula ciliaris= (or zonula Zinni) (_i_), by
which the lens is suspended.
=Chambers of the Eye.=--The lens (_h_) and zonula ciliaris (_i_) form a
partition dividing the eye into two parts. The cavity in front of the
lens contains a fluid, the =aqueous humor=. This cavity is partly
subdivided by the iris into the _anterior_ and _posterior chambers_ of
the eye.
The part of the eye lying behind the lens is much larger than the part
in front of the lens. It is lined chiefly by the retina, its front
boundary being the lens. It contains a jelly-like substance, the
=vitreous humor= (or vitreous body, =corpus vitreum=).