Chapter V: Muscles of the Leg and Foot (23)

The blood is carried away from the vascular tunic chiefly by four or five venous trunks, the =venæ vorticosæ=, which are formed by the convergence in whorls of numerous veins, coming not only from the chorioid, but also from the ciliary body and iris. The venæ vorticosæ perforate the sclera about at the equator and join the veins of the ocular muscles.

The =nerves= come from the long and short ciliary nerves. They form a plexus in the lamina suprachorioidea, which contains ganglion cells, and sends numerous non-medullated fibers chiefly to the blood-vessels of the chorioid. At the ciliary muscle a second plexus (P. gangliosus ciliaris) is formed, which supplies the muscle and sends fibers to the iris. The sphincter pupillæ is supplied by fibers derived from the oculomotor nerve, while the dilatator pupillæ is innervated by the sympathetic.

THE RETINA

FIG. 560.—INNER SURFACE OF ANTERIOR PART OF EYEBALL OF HORSE
(EQUATORIAL SECTION).

_1_, Sclera; _2_, chorioidea; _3_, retina (drawn away from
chorioidea); _4_, ciliary processes; _5_, crystalline lens, through
which the pupil (_6_) is seen. (After Ellenberger, in Leisering’s
Atlas.)
]

FIG. 561.—FUNDUS OCULI, SEEN ON EQUATORIAL SECTION OF EYEBALL OF
HORSE.

_1_, Sclera; _2_, chorioidea; _3_, retina (loosened); _4_, tapetum;
_5_, optic papilla; _6_, optic nerve. (After Ellenberger, in
Leisering’s Atlas.)
]

The =retina= or nervous tunic of the eyeball is a delicate membrane which extends from the entrance of the optic nerve to the margin of the pupil. It consists of three parts. The large posterior part, which alone contains the nervous elements, including the special neuro-epithelium and the optic nerve-fibers, is termed the =pars optica retinæ=. It extends forward to the ciliary body, where it terminates at an irregular line called the =ora serrata=. Here the retina rapidly loses its nervous elements, becomes much thinner, and is continued over the ciliary body and the posterior surface of the iris by two layers of epithelial cells. In the =pars ciliaris retinæ= the inner stratum is non-pigmented, while the outer layer is a direct continuation of the stratum pigmenti of the pars optica. In the =pars iridica retinæ= both are pigmented. In the dead subject the pars optica is an opaque, gray, soft membrane which can be stripped of the chorioid, leaving most of its outer pigmented layer on the latter. During life it is transparent, except as to its pigmented epithelium, and the reddish appearance of the fundus as viewed by the ophthalmoscope is caused by the blood in the network of the choriocapillaris. The entrance of the optic nerve forms a sharply defined, oval, light area, the =optic papilla= (Papilla nervi optici), situated about 15 mm. below the horizontal meridian and 3 to 4 mm. external to the vertical meridian. The central part of the papilla is slightly depressed (Excavatio papillæ n. optici).

The transverse diameter of the papilla is about 6 to 7 mm., and the
vertical about 4 to 5 mm. It is commonly situated a little below the
margin of the tapetum, but the latter may extend down somewhat on
either side of the papilla. The lower margin is often indented a
little. In inspection of the fundus with the ophthalmoscope numerous
fine branches of the arteria centralis retinæ are seen radiating from
the periphery of the papilla.

The optic nerve-fibers converge from all parts of the pars optica to the papilla, where they collect into bundles which traverse the lamina cribrosa of the chorioidea and sclera, and constitute the optic nerve. The =area centralis retinæ= is a round spot, 2 to 3 mm. in diameter, situated above and external to the optic papilla; it corresponds to the macula lutea of man, which is histologically more highly differentiated than the rest of the retina and is the area of most acute vision.

The =structure= of the retina is very complex. It consists of nervous elements which are supported in a peculiar sustentacular tissue, and are covered externally by a layer of pigmented epithelium (Stratum pigmenti retinæ). The nervous elements comprise a highly specialized neuro-epithelium, the rods and cones; ganglion cells, the axones of which form the optic nerve; and intermediate neurones. Ten layers may be recognized in sections microscopically.[220]

The =arteries= of the retina are derived from the arteria centralis retinæ and anastomotic branches from the short ciliary arteries. The arteria centralis enters the optic nerve a short distance behind the eyeball and runs in the axis of the nerve. The artery divides 2 to 3 mm. before reaching the papilla, and gives off thirty to forty branches which radiate in the posterior part of the retina and divide dichotomously into end-arteries. The anterior part of the retina is non-vascular. The veins accompany the arteries except in the capillary plexuses; their walls consist merely of a layer of endothelial cells, around which are a lymph-channel and sheath.

CHAMBERS OF THE EYE AND AQUEOUS HUMOR

The =anterior chamber= of the eye (Camera oculi anterior) is inclosed in front by the cornea and behind by the iris and lens (Figs. 552, 555, 557). It communicates through the pupil with the =posterior chamber= (Camera oculi posterior); this is a small annular space, triangular in cross-section, which is bounded in front by the iris, behind by the peripheral part of the lens and its ligament, and externally by the ciliary processes. The chambers are filled by the =aqueous humor= (Humor aqueus), a clear fluid which consists of about 98 per cent. of water, with a little sodium chlorid and traces of albumin and extractives. It is carried off chiefly through the spaces (of Fontana) in the zonula ciliaris or suspensory ligament of the lens into the plexus venosus scleræ.

REFRACTIVE MEDIA OF THE EYEBALL

The =vitreous body= (Corpus vitreum) is a semifluid transparent substance situated between the crystalline lens and the retina. In front it presents a deep cavity, the =fossa hyaloidea=, which receives the posterior surface of the lens. It consists of a framework of delicate fibrils, the =stroma vitreum=, the meshes of which are filled by the fluid =humor vitreus=. The surface is covered by a condensation of the stroma known as the membrana hyaloidea.

The =crystalline lens= (Lens crystallina) is a biconvex, transparent body, situated in front of the vitreous body and in partial contact with the posterior surface of the iris. Its periphery, the =æquator lentis=, is almost circular and is closely surrounded by the ciliary processes. The =anterior surface= (Facies anterior) is convex; it is bathed by the aqueous humor and is in contact with the iris to an extent which varies with the state of the pupil. The =posterior surface= (Facies posterior) is much more strongly curved than the anterior. It rests in the fossa of the vitreous body. The central points of the surfaces are the =anterior= and =posterior poles= (Polus anterior et posterior lentis), and the line which connects them is the axis of the lens (Axis lentis).

The transverse diameter of the lens is about 2 cm., the vertical
diameter is slightly smaller, and the axis measures about 13 mm. The
radius of curvature of the anterior surface is 13.5 mm., and of the
posterior surface 9.5 to 10 mm. But the curvatures of its
surfaces—especially that of the anterior—vary during life according as
the eye is accommodated for near or far vision.

The =zonula ciliaris= (of Zinn) or suspensory ligament of the lens (Fig. 552) consists of delicate fibers (Fibræ zonulares) which pass in a meridional direction from the ciliary processes to the capsule of the equator of the lens. Many fibers cross each other, and the spaces between the fibers (Spatia zonularia) are filled with aqueous humor; they communicate with each other and with the posterior chamber.

The =substance= of the lens (Substantia lentis) is inclosed by a structureless, highly elastic membrane, the =capsule= of the lens (Capsula lentis), and consists of a softer =substantia corticalis=, and a dense central part, the =nucleus lentis=. The capsule is thickest on the anterior surface, and here it is lined by a layer of flat polygonal cells, the epithelium of the lens capsule. The lens substance, when hardened, is seen to consist of concentric laminæ arranged somewhat like the layers of an onion, and united by an amorphous cement substance. The laminæ consist of lens fibers, hexagonal in section, and of very different lengths. Faint lines radiate from the poles and indicate the edges of layers of cement substance which unite the groups of lens fibers. These lines, the =radii lentis=, are three in number in the fœtus and new-born, and form with each other angles of 120 degrees. On the anterior surface one is directed upward from the pole and the other two diverge downward; on the posterior surface one is directed downward and the others diverge upward. The developed lens has neither vessels nor nerves.

In the fœtus the lens is nearly globular, and is soft and pinkish in
color. During part of fœtal life it is surrounded by a vascular
network, the tunica vasculosa lentis. This is derived chiefly from a
temporary vessel, the hyaloid artery, which is a continuation forward
of the arteria centralis retinæ through the hyaloid canal that
traverses the vitreous body. In old age the lens tends to lose its
elasticity and transparency; it also becomes flatter and the nucleus
especially grows denser.

THE EAR

The ear or organ of hearing (Organon auditus) consists of three natural divisions—external, middle, and internal.

THE EXTERNAL EAR

The =external ear= (Auris externa) comprises (1) the =auricula=, a funnel-like organ which collects the sound waves, together with its muscles, and (2) the =external auditory canal=, which conveys these waves to the tympanic membrane, the partition which separates the canal from the cavity of the middle ear.

The =auricula= or pinna is attached by its base around the external auditory canal in such a manner as to be freely movable. In the following description it will be assumed that the opening is directed outward and that the long axis is practically vertical. It has two surfaces, two borders, a base, and an apex. The convex surface or dorsum (Dorsum auriculæ) faces inward and is widest in its middle part; its lower part is almost circular in curvature, while above it narrows and flattens. The concave surface (Scapha) is the reverse of the dorsum; it presents several ridges which subside toward the apex. The anterior border is sinuous; it is largely convex, but becomes concave near the apex. It divides below into two diverging parts (crura helicis). The posterior border is convex. The apex is flattened, pointed, and curved a little forward. The base is strongly convex. It is attached to the external auditory process of the petrous temporal bone, and around this there is a quantity of fat. The parotid gland overlaps it below and externally. The structure of the external ear comprises a framework of cartilages (which are chiefly elastic), the integument, and a complicated arrangement of muscles.

The =conchal= or =auricular cartilage= (Cartilago auriculæ) determines the shape of the ear; its form can be made out without dissection, except below, where it is concealed by the muscles and the parotid gland. The basal part is coiled to form a tube, which incloses the cavity of the concha (Cavum conchæ). This part is funnel-shaped and curves outward and a little backward. Its internal (medial) surface is strongly convex, forming a prominence termed the =eminentia conchæ=. The lowest part of the internal margin bears a narrow, pointed prolongation, the =styloid process=. This process is about an inch long and projects downward externally over the annular cartilage; the guttural pouch is attached to its free end. Behind its base there is a foramen through which the auricular branch of the vagus passes.

The basal part of the posterior border is cut into by a notch, which
separates two irregular quadrilateral plates. The upper plate (Tragus)
is overlapped by the anterior border, and is separated from the
adjacent part of the posterior border (Antitragus) by a notch
(Incisura intertragica). The lower plate is curved to form a half ring
and partly overlaps the anterior border and the annular cartilage.
Behind the notch there is a foramen, which transmits the internal
auricular artery and internal auricular branch of the facial nerve.

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).