The cœliac ganglion (d) receives from the sympathetic (a), in addition to the great splanchnic nerve, two (or three) lesser splanchnic nerves (c), which arise from the main trunk of the sympathetic, the first just as it reaches the abdominal cavity, the second one or two centimeters caudad of the diaphragm.

From the cœliac and superior mesenteric ganglia a network of nerves passes in all directions to the abdominal viscera. These nerves follow especially the arteries, forming plexuses over their surfaces, and passing with them to the organs which they supply. The cœliac plexus anastomoses with the posterior gastric plexus of the vagus. Parts of the cœliac plexus following the branches of the cœliac artery may be distinguished as hepatic plexus, splenic plexus, etc. A dense plexus is formed on the surface of the suprarenal body (suprarenal plexus, j), and a renal plexus (k) follows the renal artery to the kidneys (6). The superior mesenteric plexus (g) forms a dense network over the superior mesenteric artery (5) and follows it to the intestine. A plexus passes caudad on the aorta (aortic plexus, h). This is a continuation of the cœliac and superior mesenteric plexuses; it receives a number of branches from the main sympathetic trunk, lying dorsad of the aorta. The aortic plexus contains a number of small ganglia.

From the aortic plexus the smaller inferior mesenteric plexus (i) follows the inferior mesenteric artery (7) to the large intestine; in this lies a ganglion of considerable size, the inferior mesenteric ganglion.

In the pelvic region small plexuses are formed in a similar manner, from branches given off by the main sympathetic trunk, together with branches from the plexuses of the abdominal cavity. These plexuses supply the various organs in the pelvis.

The main sympathetic trunk (a) passes into the abdomen at the side of the crus of the diaphragm, lying dorsad of the great splanchnic nerve (b). In the abdomen it lies nearer the middle line than in the thorax, so that the two trunks of right and left side are separated by but a few millimeters as they lie on the ventral surface of the centra of the lumbar vertebræ. There is a ganglion for each vertebra (Fig. 162, n), and from each ganglion are given off one or more communicating branches to the spinal nerves, and usually a branch which passes ventrad to join one of the plexuses among the viscera. The first three branches of this kind (Fig. 164, c) are larger and pass to the cœliac and superior mesenteric ganglia. The sympathetic may be traced into the pelvis, where it becomes smaller, and is lost as the caudal region is reached.

SENSE-ORGANS AND INTEGUMENT.

I. THE ORGAN OF SIGHT. ORGANON VISUS.

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).

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.

Origin on the medial surface of the concha, just caudad of the proximal end of the tragus. The fibres pass distad to be inserted on the auricular cartilage at the caudal angle of the notch which lies beneath the cranial edge of the auricle, along with the caudal fibres of the adductor auris superior (1).

Relations.—Lateral surface with the integument; medial surface with the tragus.

Action.—Draws proximad the cranial margin of the auricle.

M. antitragicus (Fig. 169, 6; Fig. 64, n).—A small muscle about one centimeter in length and three millimeters in thickness, attached at one end to the caudal border of the antitragus. The fibres pass mediodistad and are inserted on the tragus, in common with the fibres of the tragicus medialis (Fig. 169, 5, 5′).

Relations.—Outer surface with the adductor auris inferior and the integument; inner surface with the cartilage.

Action.—Constricts the external auditory opening.

M. tragicus medialis (Fig. 169, 5, 5′).—A small, flat muscle, in two parts, on the medial surface of the concha.

Origin on the ventral end of the tragus. The cranial fibres form a thick nearly cylindrical bundle which extends proximad about one centimeter, and is inserted on the cranial surface of the concha just proximad of a deep fold in the cartilage. The caudal fibres form a thin sheet which spreads on the medial surface of the concha and is inserted on an obliquely proximodistal line on the medial surface of the concha, less than one centimeter from the free distal edge.

These two bundles are distinct except at their origin, and are sometimes considered as separate muscles.

Relations.—Outer surface with the tragicus lateralis (4) and the fat about the concha. Inner surface with the concha.

Action.—Flexes the concha.

Strauss-Durckheim describes under the name cornetoconchæus a small muscle bridging the deep groove caused by the folding of the medial surface of the concha, its proximal end being close to the insertion of the abductor auris brevis (page 100). This muscle was absent in all of the cases examined for its presence.

M. conchæus externus (Fig. 169, 9).—A quadrilateral muscle on the lateral surface of the concha. The cranial end of this muscle is attached to the concha a short distance distad of the antitragus along a dorsoventral line six to eight millimeters in length; the fibres pass thence caudodorsad a distance of about one centimeter, where they are again attached to the concha. The muscle thus bridges a shallow depression on the lateral surface of the concha.

Action.—Constricts the concha.

The Middle Ear.

The middle ear is enclosed in a cavity within the tympanic bulla. Its outer boundary, as well as the inner boundary of the external ear, is formed by the tympanic membrane (Fig. 170, c.) This is a thin, semitransparent membrane attached to the bony ring (a) surrounding the inner end of the external auditory meatus. It is oval in outline. Across the dorsal third of its inner surface runs the handle of the malleus (b), to which the membrane is firmly attached and which by pulling on the membrane gives it the form of a cone instead of a flat surface. The apex of the cone is directed mediad.

The middle ear, or cavum tympani, is an ellipsoidal cavity situated in the tympanic bulla. On removing the medial wall of the bulla, a bony plate is seen separating its cavity into two chambers. This plate of bone extends in from the lateral wall of the bulla, forming a complete partition except at the dorsocaudal part, where a notch in the bone forms a free communication between the two chambers. The lateral chamber, or tympanum proper, contains the bones of the ear and is limited externally by the tympanic membrane. At its cranial end is seen the opening of the tuba auditiva, or Eustachian tube.

The tuba auditiva, or Eustachian tube, is a cartilaginous tube about one and a half to two centimeters long, passing from the nasopharynx to the middle ear and placing these two cavities in communication. The pharyngeal opening is at about the middle of the length of the nasopharynx, near the dorsal border of the lateral wall; it is a narrow slit two or three millimeters long. Thence the tube passes caudolaterad on the ventral surface of the sphenoid bone, and enters the tympanic cavity through the opening just laterad of the styliform process of the tympanic bulla. The tube has a thick, cartilaginous medial wall, while the lateral wall is thin and formed of connective tissue. The lumen of the tube is a curved slit in cross-section.

Bones of the Ear (ossicula auditus).—There are three small bones in the middle ear, the malleus (hammer) (Fig. 171, 1), incus (anvil) (Fig. 171, 2), and stapes (stirrup) (Fig. 172).

The malleus (Fig. 171, 1) consists of a slender handle (a) with a small neck (b) attached at an angle to the caudal end of the handle. At the end of the neck is a rounded head (c) which articulates with the incus (2), and together with one part of the incus lies in a small fossa dorsad of the fenestra vestibuli. From the medial aspect of the neck extends a short process of bone (d) to which is attached the tendon of the tensor tympani muscle. One side of the neck is produced into a thin wing-like plate, bearing on its edge a pointed process (e).

The incus (Fig. 171, 2) bears much resemblance to a two-fanged molar tooth. It lies in the fossa with the head of the malleus. Its head or crown is directed craniad and has a concave surface which articulates with the head of the malleus. One of the fangs (f) extends caudad in the fossa above mentioned and is held in position by a ligament. The other fang (g) extends ventromediad and articulates with the head of the stapes. This fang terminates in a minute rounded tubercle known as the lenticular process; this in early stages of development is a separate bone, the os lenticulare.