Part 37

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_).

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=.

=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=.

=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=).

II. THE EAR. ORGANON AUDITUS.

The ear of the cat, like that of man, is composed of three parts, the external, middle, and internal ear.

The =external= ear is limited internally by the tympanic membrane.

The =middle= ear extends from the tympanic membrane to the fenestra cochleæ and fenestra vestibuli. It contains the bones of the ear, and is connected with the pharynx by the Eustachian tube.

The =internal= ear is wholly contained within the petrous bone, and on the surface of the membrane lining its cavities are distributed the branches of the auditory (eighth) nerve.

=The External Ear.=

The external ear consists of an expanded portion, the =auricle= (=auricula=) or pinna, and of a canal leading from the auricle to the tympanic membrane, known as the =external auditory meatus=. The concave surface of the auricle is directed craniolaterad, and its edges form the dorsal and lateral boundaries of the auditory opening. From the bottom of the cavity which it partly encloses, the external auditory meatus passes medioventrad.

The auricle (Fig. 168) is composed of a thin curved plate of cartilage (=cartilago auriculæ=), to which numerous muscles are attached and which is covered on both sides by integument intimately attached to the cartilage. Distad the edges of the cartilage form a free border (_b_); proximad the plate is rolled together in such a way that its edges (_g_ and _h_) overlap on the craniomedial side, thus enclosing a tube or funnel (_e_) (concha) which forms the enlarged outer portion of the auditory passage.

Within the cavity of the auricle, on its medial surface, somewhat below the level of the lowest portion of the margin of the auditory opening is seen a very prominent pedunculated process (_d_). This is really part of a transverse ridge, extending craniocaudad on the medial wall of the auricular cavity. This ridge is the =antihelix= (_d_); it divides the auricula into a distal and a proximal portion. The portion distad of the antihelix is the =scapha= (_a_); its surface is smooth except for two slight longitudinal ridges extending distad from near the two ends of the antihelix. That portion of the auricle that lies proximad of the antihelix forms a deep irregular cavity known as the =concha= (_e_). A deep furrow, corresponding to the antihelix, separates scapha from concha externally. The concha extends caudad as a deep, rounded pocket; externally this pocket forms a prominent convexity, the =eminentia conchæ= (_j_), on the caudal surface of the proximal portion of the external ear. Farther proximad the concha narrows like a funnel and becomes much compressed laterally, so that a section of its cavity forms a narrow craniocaudal slit.

The inner surface of the concha is marked by a number of very prominent ridges and projections. These are usually due to folds in the cartilage, so that external furrows and depressions correspond to the internal projections. The edges of these depressions serve for attachment of muscles (Fig. 169), by contraction of which the form of the concha is changed,--the folds of the cartilage thus serving as regions of greater flexibility.

Two or three centimeters proximad of its external opening the cartilage of the concha ends in a free edge which receives within itself the distal end of the cartilaginous external auditory meatus (_e′_). The two are united together by fibrous tissue. The cartilaginous auditory meatus (_e′_) extends as a nearly cylindrical tube mediad and slightly craniad to the tympanic membrane in the opening of the bulla tympani. This tube is lined with integument which continues over the tympanic membrane. The subcutaneous tissue of the tube contains the ceruminous glands, which secrete the ear-wax and open on the surface of the skin.

To understand the relations of the muscles of the external ear it is necessary to consider a little more fully the structure of the auricular cartilage (Fig. 168), especially as compared with the human ear. The ear of the cat differs from the human ear in the fact that the edges of the auricular cartilages are rolled together proximad so as to overlap. There is thus no gap between the tragus (_g_) and antitragus (_h_), as in man, but these two structures partly overlap.

The free edge (_b_) of the auricular cartilage corresponds partly to the =helix= of man. At its mediocranial angle the proximal part of the auricle is composed of muscle and integument only, so that the cartilage when isolated presents here a deep notch distad of which is a prominent projecting angle, the =spine= (_c_) of the helix, which serves for attachment of the adductor auris superior muscle (Fig. 169, 1). Proximad of the spine, separated from it by the deep notch, the cranial edge (_g_) of the auricular cartilage approaches the caudal edge (_h_), finally overlapping it, so that the cavity of the auricle is now completely surrounded. This overlapping portion of the cartilage is on the medial side of the cavity, and in the natural state is covered by muscles and integument, so as not to be apparent.

That portion of the cranial edge of the cartilage that overlaps the caudal edge forms a cartilaginous plate which projects proximad as a blunt point; this plate is the =tragus= (_g_). Along the inner edge of the tragus is a ridge which forms a continuation of the helix; this is the =crista helicis= (_f_). The portion of the caudal edge of the cartilage that is partly overlapped by the tragus is an irregular flat plate known as the =antitragus= (_h_). At its caudolateral margin the antitragus extends distad as a thin, pointed, cartilaginous spine, the =processus uncinatus= (_i_). This supports a sheet of integument which bears two longitudinal ridges projecting into the cavity of the ear, with a broad groove between them.

=The Scutiform Cartilage.=--Closely connected with the external ear, but not forming a part of it, is the scutiform cartilage. This lies in the temporal fossa, between the integument and the temporal muscle, just behind the orbit and beneath the craniomedial portion of the auricle (Fig. 63, 1, page 97). It is a narrow cartilage about two centimeters long, with its long axis craniocaudal. For the greater part of its length it forms a slender rod, from which a thin cartilaginous sheet extends a short distance laterad; at its caudal end the rod forks, the two arms extending nearly at right angles to the main portion. The scutiform cartilage serves for the origin or insertion of a number of the muscles of the ear.

=Muscles of the External Ear= (Fig. 169).--(_a_) The muscles connecting the cartilages of the external ear with other parts of the head have been described (page 96), excepting the tragicus lateralis (4). These muscles are the following: M. intermedius scutulorum (Fig. 63, _a_), M. frontoauricularis, M. levator auris longus (including the cervicoauricular) (Fig. 63, _g_, _g′_), M. auricularis superior (Fig. 63, _k_), M. abductor auris longus (Fig. 63, _m_), M. abductor auris brevis (Fig. 63, _l_), M. zygomaticus (Fig. 64, _d_), M. submentalis (Fig. 64, _c_), M. depressor conchæ (Fig. 64, _b_), M. frontoscutularis, M. adductor auris inferior (Fig. 63, _f_), and M. tragicus lateralis (Fig. 169, 4).

=M. tragicus lateralis= (Fig. 169, 4).--A band about seven millimeters wide, lying beneath the mass of fat about the ear and running ventrad lengthwise of the concha and close against it.

_Origin_ (Fig. 96, _a_, page 229) on the caudal end of the mandibula, in the cavity found between the condyloid process and the angular process. The muscle passes dorsad and is _inserted_ (Fig. 169, 4) on the caudal margin of the tragus and in the depression on the concha just caudad of the tragus.

_Relations._--This muscle lies between the temporal muscle and the concha.

_Action._--Pulls the ear ventrad and probably rotates it outward.

(_b_) The following three muscles connect the scutiform cartilage with other parts of the external ear.

=M. rotator auris= or =scutuloauricularis inferior=.--A flat craniocaudal band of fibres lying between the auricle and the temporal muscle.

_Origin_ on the scutiform cartilage, just caudad of the insertion of the frontoscutularis, of which this muscle seems to be a continuation. The muscle passes caudad as a band seven or eight millimeters wide, curving about the medial surface of the auricle, and is inserted on the caudomedial surface of the eminentia conchæ.

_Relations._--Lateral surface with the auricular cartilage; medial surface with the temporal muscle.

_Action._--Rotates the external ear mediad and caudad.

=M. adductor auris superior= (Fig. 169, 1; Fig. 63, _f_). (Part of the auricularis anterior of man.)

_Origin_ on the dorsal surface of the scutiform cartilage for its entire length. The fibres form a continuation of those of the frontoscutularis; they pass dorsocaudad and are _inserted_ into the spina helicis or craniomedial margin of the auricular cartilage. This muscle lies in the fold of skin that connects the craniomedial angle of the auricular cartilage with the head, so that it is covered on both sides by integument.

_Action._--Draws the auricle craniad.

=M. adductor auris medius= (Fig. 169, 2). (Part of the auricularis anterior of man.)--A very thin band of fibres eight to ten millimeters broad.

_Origin_ on the middle two-thirds of the caudoventral edge of the scutiform cartilage. The fibres pass nearly ventrad and are _inserted_ along the medial or caudal surface of the tragus.

_Relations._--Outer surface with the auricular cartilage. Inner surface with the temporal muscle or with the pad of fat separating this from the auricle.

_Action._--Pulls the concha dorsocraniad.

(_c_) The remainder of the muscles of the ear interconnect parts of the auricular cartilage.

=M. transversus auriculæ= (Fig. 63, _i_).--A band about six to eight millimeters wide, on the medial convex surface of the auricle, between the distal ends of M. levator auris longus (_g′_) and M. auricularis superior (_k_).

_Origin_ on the medial surface of the concha, just proximad of the furrow which corresponds to the antihelix. The muscle bridges this furrow, passes distad about two centimeters, and is inserted on the auricular cartilage on a line which forms a caudal continuation of the line of insertion of the levator auris longus (_g′_).

_Relations._--Outer surface with the levator auris longus (_g_, _g′_), the auricularis superior (_k_), and the integument; inner surface with the auricular cartilage.

_Action._--Flexes the scapha mediad on the concha, thus enlarging the external opening of the concha.

=M. auricularis externus= (Fig. 169, 10; Fig. 64, _r_).--This consists of small scattered bands of interrupted fibres on the convex surface of the auricle, caudad of the transversus auriculæ. The largest of these bands runs parallel with the transversus auriculæ about five or six millimeters caudad of the latter. It has origin on the eminentia conchæ distad of the insertion of the abductor auris longus. Thence the fibres run distad five or six millimeters and are interrupted by an attachment to the cartilage. Distad the fibres begin again, and the muscle runs thence distad for about one centimeter, the distal end being attached to the cartilage.

Caudad of this, especially of its proximal portion, are one or two small bands of fibres usually running parallel with it, but unconnected with it and with each other. The extent and distribution of these bands varies, and the direction of fibres is also variable.

_Relations._--Outer surface with the integument; inner surface with the auricular cartilage.

_Action._--Flexes the auricular cartilage.

=M. helicis= (Fig. 169, 3; Fig. 64, _m_).--A muscle about two centimeters in length and five or six millimeters in width, on the inner surface of the ear, along its cranial margin.