Fig. 562.—Conchal and Annular Cartilages of Ear of Horse, External View.
1, Base of concha; 2, posterior border, 3, anterior border of concha; 4, intertragic notch; 5, eminentia conchæ; 6, styloid process; 7, annular cartilage; 9, zygomatic arch. (After Ellenberger-Baum, Anat. für Künstler.)
The annular cartilage (Cartilago annularis) is a quadrilateral plate, curved to form about three-fourths of a ring; its ends are a little less than half an inch (ca. 1 cm.) apart internally and are united by elastic tissue. It embraces the external auditory process and forms with the lower part of the conchal cartilage the cartilaginous part of the external auditory canal.
The scutiform cartilage (Cartilago scutiformis s. scutulum) is an irregular quadrilateral plate which lies on the temporal muscle in front of the base of the conchal cartilage. Its superficial face is slightly convex from side to side and its deep face is correspondingly concave. The anterior end is thin and rounded; the posterior part or base is wider and thicker, and its inner angle is prolonged by a pointed process half an inch or more in length. The cartilage moves very freely over the underlying parts.
The external auditory canal or meatus leads from the cavum conchæ to the tympanic membrane. It does not continue the general direction of the cavity of the concha, but extends inward, downward, and slightly forward. It consists of a cartilaginous part formed by the lower part of the conchal cartilage and the annular cartilage, and an osseous part formed by the external auditory process of the temporal bone. These are united by elastic membranes to form a complete tube. Its caliber diminishes from without inward, so that the lumen of the inner end is about half of that of the outer end.
The skin on the convex surface of the concha presents no special features; it is attached to the cartilage by a considerable amount of subcutaneous tissue except at the apex. The integument which lines the concave surface is intimately adherent to the cartilage and is relatively dark in color. There are three or four cutaneous ridges which run about parallel with the borders of the conchal cartilage, but do not extend to the apex or the cavum conchæ. The upper and marginal parts and the ridges are covered with long hairs, but between the ridges and below it is thin, covered sparsely with very fine hairs, and supplied with numerous sebaceous glands. In the external auditory canal the skin becomes thinner; in the cartilaginous part it is supplied with numerous large, coiled, ceruminous glands (Glandulæ ceruminosæ) and is sparsely covered with very fine hairs; in the osseous part the glands are small and few or absent and there are no hairs.
THE AURICULAR MUSCLES
The auricular muscles may be subdivided into two sets, viz., (a) extrinsic muscles, which arise on the head and adjacent part of the neck, and move the external ear as a whole, and (b) intrinsic muscles, which are confined to the auricula. In this connection the scutiform cartilage may be regarded as a sesamoid cartilage intercalated in the course of some of the muscles.
The extrinsic muscles are as follows:
1. The scutularis is a thin muscular sheet situated subcutaneously over the temporalis muscle. Its fibers arise from the zygomatic arch and the frontal and sagittal crests, and converge to the scutiform cartilage. It consists of three parts.
(a) The fronto-scutularis comprises temporal and frontal parts, which arise from the zygomatic arch and the frontal crest, and are inserted into the outer and anterior borders of the scutiform cartilage respectively.
(b) The interscutularis arises from the sagittal crest, over which it is in part continuous with the muscle of the opposite side. Its fibers converge to the inner border of the scutiform cartilage.
(c) The cervico-scutularis is not well defined from the preceding muscle. It arises from the occipital crest and is inserted into the inner border of the scutiform cartilage.
2. The anterior auricular muscles (Mm. auriculares nasales) are four in number.
(a) The zygomatico-auricularis arises from the zygomatic arch and the parotid fascia, and is inserted into the outer face of the base of the conchal cartilage under cover of the parotido-auricularis.
(b) The scutulo-auricularis superficialis inferior arises on the outer part of the superficial face of the scutiform cartilage and ends on the base of the conchal cartilage with the preceding muscle.
(c) The scutulo-auricularis superficialis medius arises from the posterior part of the deep surface of the scutiform cartilage and is inserted into the dorsum of the conchal cartilage, close to the lower part of its anterior border and above and behind the preceding muscle. It receives a slip from the cervico-scutularis.
(d) The scutulo-auricularis superficialis superior is a thin slip which is detached from the interscutularis over the inner border of the scutiform cartilage. It ends on the anterior aspect of the lower part of the dorsum of the conchal cartilage.
3. The superior auricular muscles (Mm. auriculares dorsales) are two in number.
(a) The scutulo-auricularis superficialis accessorius is a narrow band which is largely covered by the preceding muscle. It arises from the posterior prolongation and the adjacent part of the superficial face of the scutiform cartilage, and is inserted into the convex surface of the conchal cartilage internal to the preceding muscle, the two crossing each other at an acute angle.
(b) The parieto-auricularis is flat and triangular; it arises from the sagittal crest under cover of the cervico-scutularis, runs outward and a little backward, and is inserted by a flat tendon into the lower part of the convex surface of the concha under cover of the cervico-auricularis superficialis.
4. The posterior auricular muscles (Mm. auriculares caudales) are three in number.
Fig. 563.—External Ear of Horse and its Muscles, Lateral View.
2, Posterior border; 3, anterior border of auricula; 8, scutiform cartilage; 9, zygomatic arch; n, parotido-auricularis; o, zygomatico-auricularis; o′, scutulo-auricularis superficialis inferior; o″, scutulo-auricularis superficialis medius et superior; p, interscutularis; p′, fronto-scutularis (pars temporalis); q, cervico-auricularis profundus major; s, small (or anterior) oblique muscle of head; t, splenius; y, tendon of mastoido-humeralis; c, mastoido-humeralis; x, wing of atlas. (After Ellenberger-Baum, Anat. für Künstler.)
(a) The cervico-auricularis superficialis is a thin triangular sheet. It is wide at its origin from the occipital crest and the adjacent part of the ligamentum nuchæ, and becomes narrower as it passes outward to be inserted into the inner side of the convex surface of the concha.
(b) The cervico-auricularis profundus major arises from the ligamentum nuchæ, partly beneath and partly behind the preceding muscle. It is directed outward and is inserted into the postero-external aspect of the base of the ear, partly under cover of the parotido-auricularis.
(c) The cervico-auricularis profundus minor arises under the preceding muscle and passes downward and outward to be inserted into the lowest part of the convex surface of the concha, partly under cover of the parotid gland.
5. The parotido-auricularis (s. M. auricularis inferior) is a ribbon-like muscle which lies on the parotid gland. It is thin and wide at its origin from the fascia on the lower part of the parotid gland and becomes somewhat narrower and thicker as it passes upward. It is inserted into the conchal cartilage just below the angle of junction of its borders.
6. The deep auricular or rotator muscles (M. auriculares profundis, rotatores auriculæ), two in number, are situated under the scutiform cartilage and the base of the concha and cross each other.
(a) The scutulo-auricularis profundus major is the strongest of the auricular muscles. It is flat and is about an inch wide. It arises from the deep face of the scutiform cartilage and passes backward to end on and below the most prominent part of the base of the concha (eminentia conchæ).
(b) The scutulo-auricularis profundus minor lies between the base of the concha and the preceding muscle, and is best seen when the latter is cut and reflected. It is flat and about an inch in length. It arises from the posterior part of the deep face of the scutiform cartilage and from the cervico-scutularis, and passes downward, backward, and outward to be inserted into the base of the concha under cover of the preceding muscle.
Fig. 564.—Muscles of External Ear of Horse, Viewed from Above.
On the right side parts of the superficial muscles have been removed and the scutiform cartilage turned back to display the deeper muscles. a, Frontal part, a′, temporal part of fronto-scutularis; b, interscutularis; c, cervico-scutularis; d, e, scutulo-auricularis superficialis superior and medius; f, zygomatico-auricularis; g, scutulo-auricularis superficialis accessorius; o, cervico-auricularis superficialis; q, scutulo-auricularis superficialis inferior; t, scutiform cartilage; u, parieto-auricularis; v, cervico-auricularis profundus major; w, cervico-auricularis prof. minor; x, x′, scutulo-auricularis prof. major et minor; y, temporalis. (Ellenberger-Baum, Anat. d. Haustiere.)
7. The tragicus or mastoido-auricularis is a small muscle which arises from the temporal bone just behind the external auditory process and from the annular cartilage; it passes upward to be inserted into the lower part of the anterior border of the conchal cartilage.
The intrinsic muscles are very small and of little importance. They are as follows:
1. The antitragicus consists of a few bundles which are attached to the conchal cartilage behind the junction of its two borders, and are partially blended with the insertion of the parotido-auricularis.
2. The helicis is a small muscle attached in a position opposite to the preceding on the anterior border of the conchal cartilage; it extends also into the depression between the two divisions of the border. It is in part continuous with the insertion of the parotido-auricularis.
3. The verticalis auriculæ[221] is a thin stratum of muscular and tendinous fibers which extends upward on the posterior surface of the concha from the eminentia conchæ.
Actions.—The base of the concha is rounded and rests in a pad of fat (Corpus adiposum auriculæ), so that the movements of the external ear resemble those of a ball-and-socket joint. It is noticeable that movements about the longitudinal and transverse axes are accompanied by rotation. When the ear is vertical or drawn forward (“pricked up”), the opening is usually directed forward; conversely, when the ears are “laid back,” the opening faces backward. These movements evidently result from the coördinated actions of several muscles which are very complex and cannot be discussed in detail here. The scutularis acting as a whole fixes the scutiform cartilage, so that the muscles which arise on the latter act efficiently on the concha. The anterior auricular muscles in general erect the ear and turn the opening forward. The interscutularis concurs in this action, causing adduction and a symmetrical position of the ears; it also acts directly on the conchal cartilage, since the scutulo-auricularis superficialis superior is in reality a conchal insertion of the interscutularis. The scutulo-auricularis superficialis accessorius draws the concha forward and turns the opening outward. The parieto-auricularis adducts the concha and inclines it forward. The cervico-auricularis superficialis is chiefly an adductor of the conchal cartilage, and directs the opening outward. The cervico-auricularis profundus major turns the opening outward and, acting with the parotido-auricularis, inclines the ear toward the poll. The cervico-auricularis profundus minor tends to direct the opening downward and outward. The parotido-auricularis draws the ear downward and backward, and acts with the cervico-auricularis profundus major in “laying back the ears”; the deep auricular muscles concur in these actions. The tragicus shortens the external auditory canal. The actions of the intrinsic muscles are inappreciable.
The arteries of the external ear are derived from the anterior auricular branch of the superficial temporal, the posterior or great auricular branch of the internal maxillary, and the anterior branch of the occipital artery.
The motor nerves to the muscles come from the auricular and auriculo-palpebral branches of the facial nerve and from the first and second cervical nerves. The sensory nerves are supplied by the superficial temporal branch of the mandibular nerve and the auricular branch of the vagus.
THE MIDDLE EAR
The middle ear (Auris media) or tympanic cavity (Cavum tympani) is a space in the tympanic and petrous parts of the temporal bone situated between the membrana tympani and the internal ear. It is a pneumatic cavity, which is lined by mucous membrane, and communicates with the pharynx and the guttural pouch by the Eustachian tube. It contains a chain of auditory ossicles by which the vibrations of the membrana tympani are transmitted to the internal ear.
The cavity consists of: (1) A main part or atrium, which lies immediately to the inner side of the membrana tympani; (2) the recessus epitympanicus, situated above the level of the membrane and containing the upper part of the malleus and the greater part of the incus; (3) a relatively large ventral recess in the bulla ossea.
The external wall (Paries membranacea) is formed largely by the thin membrana tympani, which closes the inner end of the external auditory canal, and thus forms the septum between the external and middle parts of the ear. The membrane is an oval disc, which slopes obliquely downward and inward at an angle of about 30 degrees with the lower wall of the external auditory canal. The circumference is attached in a groove (Sulcus tympanicus) in the thin ring of bone (Annulus tympanicus) which almost completely surrounds it. The handle of the malleus (the outermost of the auditory ossicles) is attached to the inner surface of the membrane and draws the central part inward, producing a slight concavity of the outer surface. The periphery is thickened, forming the annulus fibrosus. The membrana tympani consists of three layers. The external cutaneous layer (Stratum cutaneum) is a prolongation of the lining of the external auditory meatus. The middle fibrous layer or membrana propria includes two chief sets of fibers; the outer stratum (Stratum radiatum) consists of fibers which radiate from the handle of the malleus, while the inner stratum (Stratum circulare) is composed of circular fibers which are best developed peripherally. There are also branched or dendritic fibers in part of the membrane. The internal mucous layer (Stratum mucosum) is a part of the general mucous membrane which lines the tympanic cavity.
Fig. 565.—Right Petrous Temporal Bone of Horse, Antero-internal View.
1, External auditory meatus; 2, annulus tympanicus; 3, laminæ; radiating from 2; 4, membrana tympani; 5, malleus; 6, incus; 7, stapes; 8, fenestra cochleæ (s. rotundum); 9, facial canal; 11, 11″, semicircular canals; 11′, vestibule; 12, cochlea. (After Ellenberger, in Leisering’s Atlas)
Fig. 566.—Right Auditory Ossicles and Membrana Tympani, Enlarged and Viewed from Inner Side and Below.
1, Malleus; 2, incus; 2′, 2″, short and long processes of 2; 3, stapes; 4, tensor tympani; 5, ligament attaching long process of malleus; 6, stapedius muscle; 7, fenestra cochleæ (s. rotundum). (After Ellenberger, in Leisering’s Atlas.)
The internal wall of the tympanic cavity (Paries labyrinthica) separates it from the internal ear; it presents a number of special features. The promontory (Promontorium) is a distinct eminence near the center which corresponds to the first coil of the cochlea, and is marked by a faint groove for the superficial petrosal nerve. Above this is the fenestra vestibuli (s. ovalis), a reniform opening which is closed by the foot-plate of the stapes and its annular ligament. The fenestra cochleæ (s. rotundum) is situated below and behind the preceding; it is an irregularly oval opening and is closed by a thin membrane (Membrana tympani secundaria), which separates the tympanic cavity from the scala tympani of the cochlea.
The anterior wall (Paries tubaria) is narrow, and is pierced by the slit-like tympanic opening of the Eustachian tube. Above this and incompletely separated from it by a thin plate of bone is the semicanal for the tensor tympani muscle.
The superior wall or roof (Paries tegmentalis) is crossed in its inner part by the facial nerve; here the facial canal is more or less deficient ventrally, and the nerve is covered by the mucous membrane of the tympanum.
The posterior wall (Paries mastoidea) presents nothing of importance; a tympanic antrum and mastoid cells, such as are found behind the tympanic cavity proper in man and many animals, are not present in the horse.
The inferior wall or floor (Paries tympanica) is concave and thin. It is crossed by delicate curved ridges, which radiate from the greater part of the annulus tympanicus.
The auditory ossicles (Ossicula auditus) form a chain which extends from the outer to the inner wall of the cavity. They are named, from without inward, the malleus, the incus, the os lenticulare, and the stapes. The first is attached to the inner surface of the tympanic membrane and the last is fixed in the fenestra vestibuli.
Fig. 567.—Auditory Ossicles and Membrani Tympani, Inner View, Enlarged.
1, Annulus tympanicus; 2, membrana tympani; 3, malleus; 4, incus; 5, stapes. (After Ellenberger, in Leisering’s Atlas.)
Fig. 568.—Right Auditory Ossicles, Enlarged as Indicated by Lines Giving their Actual Length.
a, Malleus; 1, head; 2, neck; 3, handle; 4, long process; 5, muscular process. b, Incus; 1, body; 2, short branch; 3, long branch; c, os lenticulare. c, Stapes; 1, head; 2, crus; 3, base; 4, attachment of stapedius. (After Ellenberger, in Leisering’s Atlas.)
The malleus or hammer, the largest of the ossicles, consists of a head, neck, handle, and two processes. The head (Capitulum mallei) is situated in the epitympanic recess. It is smooth and convex above and in front, and presents on its postero-internal aspect a concave facet for articulation with the body of the incus. The neck (Collum mallei) is the constricted part below the head; its inner surface is crossed by the chorda tympani. The handle (Manubrium mallei) is directed downward, inward, and a little forward from the neck, and is attached along its entire length to the membrana tympani. On its inner surface, near the upper end, there is a slight projection to which the tendon of the tensor tympani muscle is attached. The long process (Processus longus) is a pointed spicule which projects forward from the neck toward the petro-tympanic fissure. The short process (Processus brevis) is a slight projection of the outer side of the neck, and is attached to the upper part of the membrana tympani.
The incus or anvil is situated chiefly in the epitympanic recess. It may be said to resemble a bicuspid tooth with two divergent roots, and consists of a body and two processes. The body (Corpus incudis) articulates with the head of the hammer. The long process (Crus longum) projects downward from the body and then curves inward; its extremity has attached to it a small nodule of bone, the os lenticulare, which articulates with the head of the stapes. The short process (Crus breve) projects chiefly backward, and is attached to the wall of the recess by a small ligament.
The stapes or stirrup consists of a head, two crura, and a base. The head (Capitulum stapedis) is directed outward and articulates with the os lenticulare. The crura, anterior and posterior (Crus anterius, posterius), are directed inward from the head, and join the ends of the base. The base or foot-plate (Basis stapedis) occupies the fenestra vestibuli, to which it is attached. The space between the crura and the base is closed by a membrane.
The articulations and ligaments of the auditory ossicles comprise: (1) A diarthrodial joint between the head of the malleus and the body of the incus (Articulatio incudomalleolaris), inclosed by a capsule. (2) An enarthrosis between the os lenticulare and the head of the stapes (Articulatio incudostapedia), also surrounded by a capsule. (3) The base of the stapes is attached to the margin of the fenestra vestibuli by a ring of elastic fibers (Lig. annulare baseos stapedis). (4) Small ligaments attach the head of the malleus and the short crus of the incus to the roof of the epitympanic recess. (5) The axial ligament (of Helmholtz) attaches the neck of the malleus to a small projection (Spina tympanica anterior) above and in front of the annulus tympanicus.
The muscles of the auditory ossicles are two in number, viz., the tensor tympani and the stapedius. The tensor tympani arises from the upper wall of the osseous Eustachian tube, and ends in a delicate tendon which bends outward and is inserted into the handle of the malleus near its upper end. When it contracts, it draws the handle of the malleus inward and tenses the membrana tympani; it probably also rotates the malleus around its long axis. It is innervated by the motor part of the trigeminus through the otic ganglion. The stapedius arises from a small prominence (Eminentia pyramidalis) of the posterior wall of the tympanum, runs forward on the facial nerve, and is inserted into the neck of the stapes. Its action is to draw the head of the stapes backward and rotate the anterior end of the base outward, thus tensing the annular ligament. It is innervated by the facial nerve.
The tympanic mucous membrane (Tunica mucosa tympanica) is continuous with that of the pharynx and the guttural pouch through the Eustachian tube. It is thin, closely united with the underlying periosteum, and is reflected over the ossicles, ligaments, and muscles, the chorda tympani, and the facial nerve in the open part of the facial canal. It contains minute lymph nodules and small mucous glands (Glandulæ tympanicæ). The epithelium is in general columnar ciliated, but over the membrana tympani, ossicles, and promontory it is flattened.
The artery of the tympanum is the stylo-mastoid, a small vessel which arises from the posterior auricular branch of the internal maxillary artery. It enters the tympanum through the stylo-mastoid foramen, and forms a circle around the tympanic membrane. The nerves of the mucous membrane come from the tympanic plexus.
The Eustachian Tube
The Eustachian or auditory tube (Tuba auditiva [Eustachii]) extends from the tympanic cavity to the pharynx; it transmits air to the former and equalizes the pressure on the two surfaces of the membrana tympani. It is directed forward, downward, and slightly inward, and is four to five inches (ca. 10 to 12 cm.) in length. Its posterior extremity lies at the inner side of the root of the muscular process of the petrous temporal, and communicates with the anterior part of the tympanic cavity by the small slit-like tympanic opening (Ostium tympanicum tubæ auditivæ). For a distance of about a quarter of an inch (ca. 6 to 7 mm.) in front of this opening it is a complete tube, with a curved lumen which is little more than a capillary space. Further forward it has the form of a plate which widens anteriorly and is curved to inclose a narrow groove, that opens ventrally into an extensive diverticulum termed the guttural pouch. The pharyngeal opening (Ostium pharyngeum tubæ auditivæ) is situated on the postero-superior part of the lateral wall of the pharynx just below the level of the posterior nares. It is a slit, an inch or more (ca. 3 cm.) in length, which slopes downward and backward. It is bounded internally by the thin free edge of the tube, from the lower part of which a fold of mucous membrane (Plica salpingo-pharyngea) extends in the same direction on the lateral wall of the pharynx for a distance of a little more than an inch usually. The outer boundary of the opening is the lateral wall of the pharynx. The basis of the tube is a plate of fibro-cartilage (Cartilago tubæ auditivæ) which is firmly attached dorsally to the fibrous tissue which closes the foramen lacerum basis cranii, the temporal wing of the sphenoid, and the pterygoid bone.[222] On cross-section the cartilage (except at its tympanic end) is seen to consist of two laminæ which are continuous with each other above. The internal lamina (Lamina medialis) gradually widens toward the pharyngeal end, where it forms a broad valvular flap. This is convex internally, and its thin anterior edge forms the inner margin of the pharyngeal opening. Behind this the lamina has a thick free edge which projects ventrally from the roof of the guttural pouch. The external lamina (Lamina lateralis) is narrow and thin, and is related externally to the levator and tensor palati muscles, which are in part attached to it; it does not exist at the pharyngeal end of the tube. The mucous membrane of the tube is continuous behind with that of the tympanum and in front with that of the pharynx. On either side it is reflected to form a large diverticulum, the guttural pouch. It is covered with ciliated epithelium, and contains mucous glands and lymph nodules.
The pharyngeal opening appears to be closed ordinarily, but the tube opens during deglutition. This action is apparently produced by the part of the palato-pharyngeus muscle which is attached to the tube.
The Guttural Pouches
The guttural pouches, right and left (Figs. 255, 256, 569), are large mucous sacs, each of which is a ventral diverticulum of the Eustachian tube (Diverticulum tubæ auditivæ); they are not present in the domesticated animals other than the equidæ. They are situated between the base of the cranium and the atlas above and the pharynx below. Medially they are in apposition in great part, but are to some extent separated by the intervening ventral straight muscles of the head. The anterior end is a small cul-de-sac which lies below the body of the presphenoid between the Eustachian tube and the median recess of the pharynx. The posterior extremity lies near or below the atlantal attachment of the longus colli. The pouch is related dorsally to the base of the cranium, the atlanto-occipital joint capsule, and the ventral straight muscles. Ventrally it lies on the pharynx and the origin of the œsophagus. Laterally the relations are numerous and complex. They comprise the pterygoid, levator palati, tensor palati, stylo-hyoideus, and digastricus muscles; the parotid and submaxillary glands; the external carotid, internal maxillary, and external maxillary arteries; the internal maxillary and jugular veins; the pharyngeal or guttural lymph glands; the glosso-pharyngeal, hypoglossal, and superior laryngeal nerves. The vagus, accessory, and sympathetic nerves, the superior cervical ganglion, the internal carotid artery, and the inferior cerebral vein are situated in a fold of the upper wall of the pouch. The pouch is reflected around the anterior border of the great cornu of the hyoid bone so as to clothe both surfaces of the upper part of the latter. It thus forms an outer compartment, which extends backward external to the great cornu and the occipito-hyoideus muscle; it is related externally to the parotid gland, the articulation of the jaw, the ascending part of the internal maxillary artery, the superficial temporal artery, and the facial nerve; dorsally it covers the mandibular nerve and its chief branches, and is attached to the styloid process of the conchal cartilage.
Each pouch communicates with the pharynx through the pharyngeal orifice of the Eustachian tube, and is in direct continuity with the mucous membrane of the latter.
The wall of the pouch is a delicate mucous membrane which is in general rather loosely attached to the surrounding structures. It is covered with ciliated epithelium and is supplied with mucous glands.
Fig. 569.—Cross-section of Head of Horse.
The section passes through the base of the external ear and just behind the posterior border of the lower jaw. 1, Rectus capitis anterior minor; 2, rectus capitis anterior major; 3, inner walls of guttural pouches in apposition; 4, 4, arytenoid cartilage, upper piece apex; 5, posterior pillar of soft palate; 6, false vocal cord; 7, lateral ventricle of larynx; 8, true vocal cord; 9, vocal muscle; 10, thyro-hyoideus muscle.
It is worthy of note that the pharyngeal orifice of the Eustachian tube is at such a level as to provide (in the ordinary position of the head) only an overflow outlet for the escape of fluid which may accumulate in the pouch. The two pouches are often unequal in size, and variations in regard to the distance which they extend backward are not uncommon. In one case, a small aged horse, the right pouch extended along the œsophagus about five inches (ca. 12 cm.) behind the ventral tubercle of the atlas, and the left one a little more than two inches (ca. 6 cm.). No pathological changes were apparent, and the condition was not recognizable externally. Cases of extreme size—so-called tympanites—of the pouches occur, and are apparently congenital defects. In a case in a yearling colt, the head of which was 24 inches long, the left pouch extended about 12 inches (ca. 30 cm.) behind the tubercle of the atlas, and had a capacity of six quarts. The anterior end formed a cul-de-sac about two inches (ca. 5 cm.) long between the Eustachian tube and levator palati internally and the internal pterygoid muscle externally.
THE INTERNAL EAR
The internal ear or labyrinth (Auris interna s. Labyrinthus) consists of two parts, viz.: (1) a complex membranous sac, which supports the auditory cells and the peripheral ramifications of the auditory nerve; (2) a series of cavities in the petrous temporal bone, which inclose the membranous part. The first is called the membranous labyrinth, and contains a fluid, the endolymph. The second is the osseous labyrinth. The two are separated by the perilymphatic space, which is occupied by a fluid termed the perilymph.
The Osseous Labyrinth
The osseous labyrinth (Labyrinthus osseus) (Fig. 565) is excavated in the petrous temporal bone to the inner side of the tympanic cavity. It consists of three divisions: (1) a middle part, the vestibule; (2) an anterior one, the cochlea; and (3) a posterior one, the semicircular canals.
1. The vestibule (Vestibulum) is the central part of the osseous labyrinth, and communicates in front with the cochlea, behind with the semicircular canals. It is a small, irregularly ovoid cavity, which is about 5 to 6 mm. in length. Its outer wall separates it from the tympanic cavity, and in it is the fenestra vestibuli, which is occupied by the base of the stapes. The inner wall corresponds to the fundus of the internal auditory meatus. It is crossed by an oblique ridge, the crista vestibuli, which separates two recesses. The anterior and smaller of these is the recessus sphæricus, which lodges the saccule of the membranous labyrinth. In its lower part there are about a dozen minute foramina which transmit filaments of the vestibular nerve to the saccule. The posterior and larger depression is the recessus ellipticus, which lodges the utricle of the membranous labyrinth. The crista vestibuli divides below into two divergent branches, which include between them the small recessus cochlearis; this is perforated by small foramina, through which nerve-bundles reach the ductus cochlearis. Similar foramina in the recessus ellipticus and the crista vestibuli transmit nerve filaments to the utricle and the ampullæ of the superior and external semicircular ducts. The anterior wall is pierced by an opening which leads into the scala vestibuli of the cochlea. The posterior part of the vestibule presents the four openings of the semicircular canals. The inner opening of the aquæductus vestibuli is a small slit behind the lower part of the crista vestibuli. The aquæductus passes backward in the petrous temporal bone, and opens on the inner surface of the latter near the middle of its posterior border; it contains the ductus endolymphaticus.
2. The osseous semicircular canals (Canales semicirculares ossei), three in number, are situated behind and above the vestibule. They are at right angles to each other, and are designated according to their positions as superior, posterior, and external. They communicate with the vestibule by four openings only, since the inner end of the superior and the upper end of the posterior canal unite to form a common canal (Crus commune), and the ampullate ends of the superior and external canals have a common orifice. Each canal forms about two-thirds of a circle, one end of which is enlarged and termed the ampulla. The superior canal (Canalis semicircularis superior) is nearly vertical and is placed obliquely with regard to a sagittal plane, so that its outer limb is further forward than the inner one. The antero-external end is the ampulla and opens into the vestibule with that of the external canal. The opposite non-dilated end joins the adjacent end of the posterior canal to form the crus commune, which opens into the supero-internal part of the vestibule. The posterior canal (Canalis semicircularis posterior) is also nearly vertical. Its ampulla is below, and opens into the vestibule directly, while the non-dilated end joins that of the superior canal. The external canal (Canalis semicircularis lateralis) is nearly horizontal. Its ampulla is external and opens into the vestibule with that of the superior canal.
3. The cochlea is the anterior part of the bony labyrinth. It has the form of a short blunt cone, the base of which (Basis cochleæ) corresponds to the anterior part of the fundus of the internal auditory meatus, while the apex or cupola (Cupula) is directed outward, forward, and downward. It measures about 7 mm. across the base and about 4 mm. from base to apex. It consists of a spiral canal (Canalis spiralis cochleæ), which forms two and a half turns around a central column termed the modiolus. The modiolus diminishes rapidly in diameter from base to apex. Its base (Basis modioli) corresponds to the area cochleæ of the fundus of the internal auditory meatus, and its apex extends nearly to the cupola. Projecting from the modiolus like the thread of a screw is a thin plate of bone, the lamina spiralis ossea. This begins between the two fenestræ and ends near the cupola as a hook-like process (Hamulus laminæ spiralis). The lamina extends about half-way to the periphery of the cochlea and partially divides the cavity into two passages; of these, the upper one is termed the scala vestibuli, and the lower the scala tympani. The membrana basilaris extends from the free margin of the lamina to the outer wall of the cochlea and completes the septum between the two scalæ, but they communicate through the opening (Helicotrema) at the cupola. The modiolus is traversed by an axial canal which transmits the nerves to the apical coil, and by a spiral canal (canalis spiralis modioli), which follows the attached border of the lamina spiralis, and contains the spiral ganglion and vein. Close to the beginning of the scala tympani is the inner orifice of the aquæductus cochleæ, a small canal which opens behind the internal auditory meatus, and establishes a communication between the scala tympani and the subarachnoid space.
The internal auditory meatus has been described in part (vide Osteology). The fundus of the meatus is divided by a ridge (Crista transversa) into upper and lower parts. The anterior part of the upper depression (Area n. facialis) presents the cranial opening of the facial canal; and the posterior part (Area vestibularis superior) is perforated by foramina for the passage of nerves to the utricle and the ampullæ of the superior and external membranous semicircular canals. The anterior part of the inferior depression (Area cochleæ) presents a central foramen and a spiral tract of minute foramina (Tractus spiralis foraminosus) for the passage of nerves to the cochlea. Behind these is an area of small openings which transmit nerves to the saccule (Area vestibularis inferior), and a single foramen (F. singulare) for the passage of a nerve to the ampulla of the posterior semicircular canal.
The Membranous Labyrinth
The membranous labyrinth (Labyrinthus membranaceus) lies within, but does not fill, the osseous labyrinth. It is attached to the latter by delicate trabeculæ which traverse the perilymphatic space. It conforms more or less closely to the bony labyrinth, but consists of four divisions, since the vestibule contains two membranous sacs—the utricle and saccule.
1. The utricle (Utriculus), the larger of the two sacs, lies in the postero-superior part of the vestibule, largely in the recessus ellipticus. It receives the openings of the membranous semicircular canals, and the small ductus utriculo-saccularis leads from its lower part to the ductus endolymphaticus.
2. The saccule (Sacculus) is situated in the recessus sphæricus of the vestibule. From its lower part the ductus reuniens proceeds to open into the ductus cochlearis, a little in front of the blind end of the latter. A second narrow tube, the ductus endolymphaticus, passes from the posterior part of the saccule, and is joined by the ductus utriculo-saccularis; it then traverses the aquæductus vestibuli, and terminates under the dura mater on the posterior part of the internal surface of the petrous temporal bone in a dilated blind end, the saccus endolymphaticus.
3. The membranous semicircular canals (Ductus semicirculares) correspond in general to the osseous canals already described, but it may be noted that while the ampullæ of the membranous canals nearly fill those of the osseous canals, the other parts of the membranous canals only occupy about one-fourth of the bony cavities.
4. The cochlear duct (Ductus cochlearis) is a spiral tube situated within the cochlea. It begins by a blind end (Cæcum vestibulare) in the cochlear recess of the vestibule, and ends by a second blind end (Cæcum cupulare), which is attached to the cupola of the cochlea. The vestibular part is connected with the saccule by the ductus reuniens. The duct is triangular in cross-section, and it is usual to regard it as having three walls. The vestibular wall or roof, which separates the cochlear duct from the scala vestibuli, is formed by the very delicate membrana vestibularis (of Reissner), which extends obliquely from the lamina spiralis ossea to the outer wall of the cochlea. The tympanic wall or floor intervenes between the cochlear duct and the scala tympani; it is formed by the periosteum of the marginal part of the lamina spiralis and the membrana basilaris, which stretches between the free edge of the lamina and the outer wall of the cochlea. The outer wall consists of the fibrous lining of the cochlea, which is greatly thickened to form the ligamentum spirale cochleæ.
Fig. 570.—Left Membranous Labyrinth (Enlarged).
1, Cochlea; 2, fenestra vestibuli; 3, fenestra cochleæ; 4, ductus endolymphaticus; 5, superior, 6, external, 7, inferior semicircular canal. (After Ellenberger, in Leisering’s Atlas.)
Fig. 571.—Schematic Sectional View of Labyrinth (Enlarged).
1, 2, 3, Superior, external, and inferior semicircular canals; 4, utricle; 5, saccule; 6, cochlea; 7, auditory nerve. (After Ellenberger, in Leisering’s Atlas.)
Structure.[223]—The membranous labyrinth consists in general of an outer thin fibrous layer, a middle transparent tunic, and an internal epithelium, composed of flattened cells. But in certain situations special and remarkable structures occur, among which are the following: (1) The maculæ acusticæ appear as small whitish thickenings of the inner walls of the saccule and utricle. The epithelium here consists of two kinds of cells, viz., supporting cells and hair cells. The latter are flask-shaped and are surrounded by the fusiform supporting cells. The free end of each hair cell bears a stiff, hair-like process composed of a bundle of cilia. Fibers of the saccular and utricular branches of the vestibular nerve form arborizations about the basal parts of the hair cells. Adherent to the surface of the maculæ are fine crystals of lime salts, embedded in a mucoid substance, and termed otoconia. (2) The cristæ acusticæ are linear thickenings of the wall of each ampulla of the membranous semicircular canals. Their structure is similar to that of the maculæ. (3) The spiral organ of Corti (Organon spirale) is an epithelial elevation which is situated upon the inner part of the membrana basilaris, and extends the entire length of the ductus cochlearis. It is very complicated in structure, but consists essentially of remarkable supporting cells and hair cells. Fibers of the cochlear nerve ramify about the basal parts of the hair cells.
Vessels and Nerves.—The artery of the internal ear is the internal auditory artery, a very small vessel which usually arises from the posterior cerebellar artery, and enters the internal auditory meatus. The veins go to the inferior petrosal sinus. The vestibular nerve is distributed to the utricle, saccule, and membranous semicircular canals, and mediates equilibration. The cochlear nerve gives a branch to the saccule and enters the central canal of the modiolus. Along its course it gives off fibers which radiate outward between the two plates of the lamina spiralis ossea, and ramify about the hair cells of the organ of Corti. The ganglion spirale or ganglion of Corti is situated in the spiral canal of the modiolus near the fixed border of the lamina spiralis. The cochlear nerve mediates the sense of hearing.
The Skin
The skin or common integument (Integumentum commune) is the protective covering of the body, and is continuous at the natural openings with the mucous membranes of the digestive, respiratory, and urogenital tracts. It contains peripheral ramifications of the sensory nerves, and is thus an important sense organ. It is the principal factor in the regulation of the temperature of the body, and by means of its glands it plays an important part in secretion and excretion. Some of its special horny modifications or appendages are used as organs of prehension or as weapons.
The thickness of the skin varies in the different species, on different parts of the body of the same animal, and also with the breed, sex, and age. The color also varies greatly, but this is masked in most places by the covering of hair or wool. The skin is in general highly elastic and resistant.
Permanent folds of the skin (Plicæ cutis) occur in certain situations, and in some places there are cutaneous pouches or diverticula (Sinus cutis).
The skin is attached to the underlying parts by the subcutis or superficial fascia (Tela subcutanea). This consists of connective tissue containing elastic fibers and fat. When the fat forms a layer of considerable thickness, it is termed the panniculus adiposus. Over a considerable part of the body the subcutis contains striped muscle, the panniculus carnosus (M. cutaneus); in some regions the fibers of the muscle are inserted into the skin, and their contraction twitches the skin or produces temporary folds.[224] The amount of subcutaneous tissue varies widely; in some places it is abundant, so that the skin can be raised considerably; in other situations it is practically absent and the skin is closely adherent to the subjacent structures. Subcutaneous bursæ often develop over prominent parts of the skeleton where there is much pressure or friction.
Structure.—The integument consists of the cutis or skin proper and its epidermal appendages, e. g., hairs, hoofs, claws, horns, etc. The cutis consists of two distinct strata, viz., a superficial epithelial layer, the epidermis, and a deep connective-tissue layer, the corium.
The epidermis is a non-vascular, stratified epithelium of varying thickness. It presents the openings of the cutaneous glands and the hair-follicles, and its deep surface is adapted to the corium.[225] It is divisible into a superficial, harder, drier part, the stratum corneum, and a deeper, softer, moister part, the stratum germinativum. The cells of the latter contain pigment, and by their proliferation compensate the loss by desquamation of the superficial part of the stratum corneum. In many places further subdivision into strata is evident on properly prepared cross-sections.
The corium consists essentially of a feltwork of connective-tissue and elastic fibers. It is well supplied with vessels and nerves, and contains the cutaneous glands, the hair-follicles, and unstriped muscle. The deeper part of the corium, the tunica propria, consists of a relatively loose network of coarse bundles of fibers, and in most places there is no clear line of demarcation between it and the subcutis. The superficial part, the corpus papillare, is of finer texture and is free from fat. Its superficial face is thickly beset with blunt conical prominences, the papillæ, which are received into corresponding depressions of the epidermis. They contain vascular loops and nerves, or, in certain situations, special nerve-endings.
The papillæ are best developed where the epidermis is thick and hairs are small or absent. On thickly-haired regions they are small or even absent. On certain parts of the body (anus, vulva, prepuce, scrotum, eyelids, etc.) the corium contains pigment in its connective-tissue cells.
The glands of the skin (Glandulæ cutis) are chiefly of two kinds, sudoriferous and sebaceous.
The sudoriferous or sweat glands (Glandulæ sudoriferæ) consist of a tube, the lower, secretory part of which is coiled in the deep part of the corium or in the subcutis to form a round or oval ball (Corpus glandulæ sudoriferæ). The excretory duct (Ductus sudoriferus) passes almost straight up through the corium, but pursues a more or less flexuous course through the epidermis, and opens into a hair-follicle or by a funnel-shaped pore (Porus sudoriferus) on the surface of the skin.
The sebaceous glands (Glandulæ sebaceæ) are in great part associated with the hairs, into the follicles of which they open. Their size varies widely, and is in general in inverse ratio to that of the hair. The larger ones are easily seen with the naked eye, and appear as small, pale yellow or brownish bodies. In certain situations (e. g., the labia vulvæ, anus, prepuce) they are independent of the hairs and are well developed. In form they may be branched alveolar, simple alveolar, or even tubular in type. They secrete a fatty substance, the sebum cutaneum, which serves as a protective against moisture, and may also (by its aromatic constituents) play an important part in the sexual life of animals.
The two kinds of glands described above are those which are most widely distributed, but many special types occur. Some of these are to be regarded as modified sweat glands, e. g., the naso-labial glands of the ox, the glands of the snout of the pig, and the glands of the plantar cushion of the horse. Others, e. g., the tarsal (or Meibomian) glands of the eyelids, are modified sebaceous. Still others are not yet classified satisfactorily. Some of these special types have been referred to in previous chapters, and others will receive attention in the special descriptions which follow. The mammary glands are highly modified cutaneous glands, which are intimately associated in function with the genital organs, and have been described with the latter.
Vessels and Nerves.—The arteries of the skin enter from the subcutis, where they communicate freely. In the deeper part of the corium they form a plexus, and another network is formed under the papillæ. Small vessels from the deep plexus go to the fat and sweat glands, and the subpapillary plexus sends fine branches to the papillæ, hair-follicles, and sebaceous glands. The veins form two plexuses, one beneath the papillæ, and another at the junction of the corium and subcutis. The lymph vessels form subpapillary and subcutaneous plexuses.
The nerves vary widely in number in different parts of the skin. The terminal fibers either end free in the epidermis and in certain parts of the corium, or form special microscopic corpuscles of several kinds.
THE APPENDAGES OF THE SKIN
The appendages of the skin are modifications of the epidermis, and comprise the hairs, hoofs, claws, horns, etc.
The hairs (Pili) cover almost the entire surface of the body in the domesticated mammals, and some parts which appear at first sight to be bare are found on close inspection to be provided with sparse and very fine hair. The hairs are constantly being shed and replaced, but at certain periods in the horse, for example, they fall out in great numbers, constituting the shedding of the coat. It is customary to distinguish the ordinary hairs (the coat), which determine the color of the animal, from the special varieties found in certain places. Among the latter are the long tactile hairs about the lips, nostrils, and eyes; the eyelashes or cilia; the tragi of the external ear; and the vibrissæ of the nostrils. Other special features will be noted in the discussion of the skin of the various species. The hairs are directed in such a way as to form more or less definite hair-streams (Flumina pilorum), and at certain points these converge to form vortices (Vortices pilorum).