A text-book of veterinary anatomy

This system consists of the organs directly concerned in the reception and digestion of the food, its passage through the body, and the expulsion of the unabsorbed portion. These organs are conveniently grouped under two heads, viz.: (1) the alimentary canal; (2) the accessory organs.

The alimentary canal (Tractus alimentarius) is a tube, about 100 feet (ca. 30 m.) in length in the horse, and extends from the lips to the anus. It has a complete lining of mucous membrane, external to which is an almost continuous muscular coat. The abdominal portion of the tube is largely covered with a serous membrane—the visceral peritoneum. The canal consists of the following consecutive segments:

1.

Mouth.

2.

Pharynx.

3.

Œsophagus.

4.

Stomach.

5.

Small intestine.

6.

Large intestine.

The accessory organs are the teeth, tongue, salivary glands, liver, and pancreas.

DIGESTIVE SYSTEM OF THE HORSE

THE MOUTH

The mouth^[77] (Cavum oris) is the first part of the alimentary canal. In the horse it is a long cylindrical cavity, and when closed, it is almost entirely filled up by the contained structures. The entrance to it (Rima oris) is closed by the lips. Laterally it is bounded by the cheeks; above, by the hard palate; below, by the body of the mandible and the mylo-hyoid muscles; behind, by the soft palate.

The cavity of the mouth is subdivided into two parts by the teeth and alveolar processes. The space external to these and inclosed externally by the lips and cheeks is termed the vestibule of the mouth (Vestibulum oris). In the resting state of the parts the walls of this cavity are in contact, and the space is practically obliterated. Its existence becomes very evident in facial paralysis, when the food tends to collect in it laterally, pouching out the cheeks. The space within the teeth and alveolar processes is termed the mouth cavity proper (Cavum oris proprium). When the teeth are in contact, it communicates with the vestibule only by the interdental spaces and the intervals behind the last molar teeth. Posteriorly it communicates with the pharynx through the isthmus of the fauces.

The mucous membrane lining the mouth (Tunica mucosa oris) is continuous at the margin of the lips with the common integument, and behind with the mucous lining of the pharynx. During life it is chiefly of a pink color.

The lips (Labia oris) are two musculo-membranous folds which surround the orifice of the mouth. Their angles of union (Anguli oris s. commissuræ labiorum) are situated near the first cheek tooth and are rounded. Each lip presents two surfaces and two borders. The outer surface is covered by the skin, which presents long tactile hairs in addition to the ordinary fine hair. The upper lip shows a shallow median furrow (Philtrum), the lower a rounded prominence, the chin (Mentum). The inner or oral surface is covered with mucous membrane which is commonly more or less pigmented. The small papillæ on this surface show on their summits the openings of the ducts of the labial glands. Small folds of mucous membrane which pass from the lip to the gum represent the frænula labii (superioris, inferioris). The free border of the lip is dense and presents short, very stiff hairs. The attached border is continuous with the surrounding structures, and is adherent to the alveolar borders of the bones of the jaws.

Structure.—The lips are covered externally by the skin, and are lined by mucous membrane; between these are muscular tissue and glands. The skin lies directly on the muscles, many fibers of which are inserted into the former. The muscles have been described (page 214). The labial glands (Glandulæ labiales) form a compact mass near the angles; they are numerous in the upper lip, fewer in the lower. The mucous membrane is often pigmented, and is reflected upon the bones of the jaws to form the gums.

Blood-vessels and Nerves.—The arteries are derived from the superior and inferior labial and palato-labial arteries. The sensory nerves come from the trigeminus, and the motor from the facial nerve.

The cheeks (Buccæ) form the sides of the mouth, and are continuous in front with the lips. They are attached above and below to the alveolar borders of the bones of the jaws.

Structure.—This comprises: (1) The skin; (2) the muscular and glandular layer; (3) the mucous membrane. The skin offers nothing in particular to notice. The muscular tissue is formed mainly by the buccinator, but also by parts of the panniculus, zygomaticus, dilatator naris lateralis, levator nasolabialis, and depressor labii inferioris. The buccal glands (Glandulæ buccales) are arranged in two rows. The upper row (superior buccal or molar glands) is found on the outer surface of the buccinator muscle, near its upper border. The anterior part of the row consists of scattered lobules; the posterior part, which lies under cover of the masseter muscle, is more developed and compact. The lower row (inferior buccal or molar glands), less voluminous than the upper, is situated in the submucous tissue at the lower border of the buccinator muscle. The mucous membrane is reflected above and below upon the gums, and is continuous behind with that of the soft palate. It is reddish in color and frequently shows pigmented areas. Opposite the third upper cheek tooth is the opening of the parotid (Stenson’s) duct, surrounded by a circular fold of the mucous membrane. A linear series of small papillæ above and below indicates the orifices of the small ducts from the buccal glands.

Blood-vessels and Nerves.—The blood-supply is derived from the facial and buccinator arteries. The sensory nerves come from the trigeminus and the motor from the facial nerve.

The gums (Gingivæ) are composed of a dense fibrous tissue intimately united with the periosteum of the alveolar processes, and blending at the edges of the alveoli with the alveolar periosteum, which fixes the teeth in their cavities. They are covered by a smooth mucous membrane, destitute of glands, and of a low degree of sensibility.

The hard palate (Palatum durum) is bounded in front and on the sides by the alveolar arches; behind it is continuous with the soft palate. Its osseous basis is formed by the premaxilla, maxilla, and palate bones. The mucous membrane is smooth, and is attached to the bones by a submucosa which contains in its anterior part an exceedingly rich venous plexus, constituting an erectile tissue. A central raphé (Raphé palati) divides the surface into two equal portions. Each of these presents about eighteen transverse curved ridges (Rugæ palatini) which have their concavity and their free edges directed backward. They are further apart and more prominent anteriorly. There are no glands in the submucosa.

Vessels and Nerves.—The blood-supply is derived chiefly from the palatine arteries and the nerves from the trigeminus.

The soft palate (Palatum molle) is a musculo-membranous curtain which separates the cavity of the mouth from that of the pharynx. It slopes downward and backward from its junction with the hard palate. The oral surface faces downward and forward, and is covered with a mucous membrane continuous with that of the hard palate. It is corrugated and presents numerous small orifices (of gland-ducts) and two sagittal ridges. On each side a short, thick fold passes to join the base of the tongue; this is the anterior pillar of the soft palate (Arcus glossopalatinus). The pharyngeal surface looks upward and backward and is covered by a mucous membrane continuous with that of the nasal cavity. The free border (Arcus palatinus) is concave and thin; it is in contact (except during deglutition) with the epiglottis. It is continuous with a fold of the mucous membrane, which passes on each side along the lateral wall of the pharynx and unites with its fellow over the beginning of the œsophagus; this fold is termed the posterior pillar of the soft palate (Arcus pharyngopalatinus). The space between the diverging anterior and posterior pillars (Sinus tonsillaris) is occupied by the faucial tonsil. In the horse, however, there is not a compact tonsil, as in man, dog, etc., but a somewhat extended group of mucous glands and masses of lymphoid tissue. These cause elevations of the surface, on which are seen numerous depressions (crypts) in which the gland-ducts open. The soft palate is greatly developed in equidæ, its length being five to six inches (12 to 15 cm.). Its length and contact with the epiglottis account for the fact that in these animals mouth-breathing does not occur under normal conditions, and that in vomiting the ejected matter escapes usually through the nasal cavity.^[78]

Structure.—The soft palate consists of: (1) The oral mucous membrane, continuous with that of the hard palate; (2) the palatine glands (Glandulæ palatinæ), which form a layer about half an inch in thickness; (3) the aponeurotic and muscular layer; (4) the pharyngeal mucous membrane, continuous with that of the nasal cavity.

The muscles proper to the soft palate are the azygos uvulæ, the levator palati, and the tensor palati.

The azygos uvulæ (M. palatinus) consists of two small muscular bundles which lie together at the median line. It is attached through the medium of the palatine aponeurosis to the palatine arch, and terminates near the free edge of the soft palate. Its action is to shorten and raise the soft palate.

The levator palati (M. levator veli palatini) arises from the muscular process of the petrous temporal bone and from the Eustachian tube, and passes forward and downward externally to the latter, to spread out on the pharyngeal surface of the soft palate. It raises the soft palate, thus closing the posterior nares during deglutition.

The tensor palati (M. tensor veli palatini) is larger than the levator, and is fusiform and flattened. It arises from the muscular process of the petrous temporal bone and the Eustachian tube, and passes forward external to the levator. Its tendon is then reflected around the hamulus of the pterygoid bone, being held in position by a fibrous band and lubricated by a bursa. It turns inward and expands in the aponeurosis of the soft palate. It tenses the soft palate.

Vessels and Nerves.—The blood-supply of the soft palate is derived from the internal and external maxillary arteries. The nerves come from the trigeminus, vagus, and glosso-pharyngeal nerves.

The floor of the mouth in its anterior part is free and is formed by the body of the mandible, covered by mucous membrane. The remainder is concealed by the attached portion of the tongue, with the exception of a narrow space on each side of the latter. About opposite the canine tooth on each side is a papilla, the caruncula sublingualis, through which the duct of the submaxillary gland opens. Just behind these papillæ is a median fold of mucous membrane which passes to the under surface of the tongue, constituting the frenum linguæ. On either side are the sublingual crests, which extend from the frenum to the level of the fourth cheek tooth. The crest presents numerous small papillæ, through which open the ducts from the subjacent sublingual gland.^[79] Behind the last tooth a vertical fold of the mucous membrane passes from upper to lower jaw. This is termed the plica pterygomandibularis: it contains a ligament of like name.

The isthmus faucium is the orifice of communication between the mouth and the pharynx. It is bounded above by the soft palate, below by the root of the tongue, and laterally by the anterior pillars of the soft palate. It is long, relatively small, and not very dilatable in the horse, and is closed by the soft palate under normal conditions, except during deglutition.

THE TONGUE

The tongue (Lingua) is situated on the floor of the mouth, between the rami of the mandible, and is supported mainly in a sort of sling formed by the mylo-hyoid muscles. Its posterior portion, the root (Radix linguæ), is attached to the hyoid bone, soft palate, and pharynx. Only the upper surface of this part is free, and slopes downward and backward. The middle portion, the body (Corpus linguæ), has three free surfaces. The upper surface or dorsum (Dorsum linguæ) is rounded. The lateral surfaces are nearly flat for the most part, but anteriorly become rounded and narrower. The lower surface is attached to the mandible. The apex or tip (Apex linguæ) is free, spatula-shaped, and presents superior and inferior surfaces and a rounded border.

Structure.—The tongue consists of: (1) The mucous membrane; (2) the glands; (3) the muscles.

The mucous membrane (Tunica mucosa linguæ) adheres intimately to the subjacent tissue, except on the lower part of the lateral surfaces of the body and the under surface of the tip. It varies considerably in thickness. On the dorsum it is very thick and dense. Underneath this portion there is a dense fibrous cord, which extends medially a distance of five or six inches forward from the vallate papillæ. On the sides and under surface of the tongue the membrane is much thinner and smooth, and can more readily be dissected off the muscular tissue. From the under surface of the free part of the tongue a fold of the mucous membrane passes to the floor of the mouth, forming the frenum linguæ. This contains the anterior edges of the genio-glossi muscles. Posteriorly a fold passes on each side from the edge of the dorsum to join the soft palate, forming the anterior pillars of the latter. A central glosso-epiglottic fold (Plica glossoepiglottica) passes from the root to the base of the epiglottis. The mucous membrane presents numerous papillæ, which are of four kinds—filiform, fungiform, vallate, and foliate. The filiform papillæ (Papillæ filiformes) are fine, pointed projections. They cover the upper surface of the body and tip, to which they give a distinct pile. The fungiform papillæ (Papillæ fungiformes) are rounded at the free end, which is supported by a neck. They occur principally on the lateral part of the tongue, but are also found scattered over the dorsum and upper surface of the free portion. The vallate papillæ (Papillæ vallatæ) are usually two or three in number. The two constant ones have a diameter of about 7 mm., and are found on the posterior part of the dorsum, one on each side of the median plane, about an inch (ca. 3 cm.) apart. The third, when present, is behind these, is centrally situated, and is always smaller. Rarely a fourth may be seen. They are rounded, broader at their exposed than at their attached surfaces, and are situated in a cup-shaped cavity. The foliate papillæ (Papillæ foliatæ) are situated just in front of the anterior pillars of the soft palate, where they form a rounded eminence about an inch (ca. 2 to 3 cm.) in length, marked by transverse fissures. The last three varieties are covered with microscopic secondary papillæ and are furnished with taste buds. The mucous membrane of the root of the tongue presents numerous folds and depressions. Into the latter open the ducts from the lingual glands (Glandulæ linguales), which constitute a thick layer in the loose submucous tissue. Mucous glands are found also on the dorsum and sides of the tongue. In the submucosa of the root is found also a large quantity of lymph follicles (Folliculi tonsillares) and diffuse lymphoid tissue.

The muscular tissue may be divided into intrinsic and extrinsic. The intrinsic musculature consists, not of distinct muscles, but rather of systems of fibers which run longitudinally, vertically, and transversely, blending with the extrinsic muscles, which are now to be described.

1. Stylo-glossus.—This is a long, thin muscle, which lies on the lateral part of the tongue. It arises by a thin tendon from the outer surface of the great cornu of the hyoid bone, near the articulation with the small cornu. It terminates near the tip of the tongue by blending with its fellow of the opposite side and with the intrinsic musculature. The action is to retract the tongue. Unilateral contraction would also draw the tongue toward the side of the muscle acting.

2. Hyo-glossus.—This is a wide, flat muscle, somewhat thicker than the preceding. It lies on the lateral part of the root and body of the tongue, partly under cover of the preceding muscle. Its deep face is related to the genio-glossus. It arises from the lateral aspect of the hyoid bone, from the lingual process to the oral extremity of the great cornu, and from the thyroid cornu. The fibers pass obliquely forward and upward, and for the most part turn toward the median plane of the dorsum of the tongue. Its action is to retract and depress the tongue.

(It is usually possible to recognize in this muscle three portions, which would correspond to the baseo-, kerato-, and chondro-glossus of human anatomy.)

3. Genio-glossus.—This is a fan-shaped muscle, which lies parallel to the median plane of the tongue. It is separated from the muscle of the opposite side by a quantity of fat and connective tissue. It arises from the inner surface of the ramus of the mandible near the symphysis. From the tendon the fibers pass in a radiating manner, some toward the tip, others toward the dorsum, and others toward the root of the tongue; some fibers pass from the posterior end of the tendon to the body and small cornu of the hyoid bone. The muscle as a whole is a depressor of the tongue, and especially of its middle portion; when both muscles act, a median groove is formed on the dorsum. The posterior fibers protrude the tongue, the middle fibers depress the tongue, and the anterior fibers retract the tip of the tongue.

Vessels and Nerves.—The arteries of the tongue are the lingual and sublingual branches of the external maxillary artery. The sensory nerves are the lingual and glosso-pharyngeal, and the muscles are innervated by the hypoglossal nerve.

THE TEETH

The teeth are hard white or yellowish-white structures, implanted in the alveoli of the bones of the jaws—premaxilla, maxilla, and mandible. Morphologically they are large calcified papillæ. Functionally they are organs of prehension and mastication, and may serve as weapons of offense and defense. They are classified according to form and position as follows:

1. The incisor teeth (Dentes incisivi) are situated in front and are implanted in the premaxilla and mandible.

2. The canine teeth (Dentes canini) are situated a little further back, at or near the premaxillary suture in the upper jaw; in the lower jaw they are nearer the incisors.

3. The cheek teeth (Dentes premolares et molares), the remaining teeth, occupy the sides of the dental arch. The space between the incisors and cheek teeth is termed the interdental or interalveolar space. It is customary to divide the cheek teeth into an anterior series, termed premolars, which appear as temporary, deciduous, or milk teeth, and are replaced by permanent successors, and a posterior series, the molars, which appear only as permanent teeth without deciduous predecessors.

As the teeth of the two sides of the jaws are alike in number and character (in normal cases), the complete dentition may be briefly indicated by a dental formula such as the following:

In this formula the letters indicate the kinds of teeth, and the figures above and below the lines give the number of teeth of one side in the upper and lower jaw respectively in man.

The individual teeth of each group are designated numerically, the starting-point being the middle line; thus the incisor on either side of the middle line is the first incisor, and may be conveniently indicated by the notation I¹. The temporary or deciduous teeth may be designated in a similar manner, prefixing D (for deciduous) to the letter indicating the kind of tooth. In addition to the above systematic method of notation other terms have received the sanction of popular usage. Thus the first incisors are commonly called “pinchers,” or “nippers”; the second, intermediate; and the third, corner teeth. The canines, when highly developed, may be termed tusks or fangs. The vestigial and inconstant first premolar of the horse is popularly termed the “wolf tooth.”

Each tooth presents for description a portion coated with enamel, termed the crown (Corona dentis), and a portion covered with cement, termed the root (Radix dentis). The line of union of these parts is the neck (Collum dentis).^[80] In many teeth the neck is distinct and is embraced by the gum, e. g., the teeth of the dog and the temporary incisors of the horse. In other teeth no constriction is seen, as in the permanent incisors of the horse. Between these extremes may be noted the molars of the horse, in which the neck is seen only in advanced age.

The surface of a tooth directed toward the lips is termed labial; toward the cheek, buccal; and toward the tongue, lingual (Facies labialis, buccalis, lingualis). The surface opposed to a neighboring tooth of the same dental arch is termed the contact surface (Facies contactus). The grinding or masticatory or “table” surface (Facies masticatoria) is that which comes in contact with a tooth or teeth of the opposite jaw.

Structure.—Teeth are composed of four tissues, which are considered here from within outward. The pulp (Pulpa dentis) is a soft gelatinous tissue, which occupies a space in the central part of the tooth termed the pulp cavity (Cavum dentis). The pulp is well supplied with blood-vessels and nerves. It occupies a relatively large space in young growing teeth, but later the dentine deposited on its surface gradually encroaches on it until, in advanced age, the cavity is obliterated or much reduced. The dentine (Substantia eburnea) forms the bulk of most teeth, covering the surface of the pulp. It is very hard, and is yellowish-white in color. The enamel (Substantia adamantina) constitutes a layer of varying thickness covering the dentine of the crown of the tooth. It is easily distinguished by its clear bluish-white appearance and its extreme density. The cement (Substantia ossea) is always the outermost tooth substance. In simple teeth it forms usually a thin layer on the surface of the dentine of the root only, but in complex teeth it exists in considerable quantity, tending to fill in the spaces between the enamel folds of the crown also. Its structure is practically the same as that of bone without Haversian canals, and even these occur where the cement forms a very thick layer. The embedded part of the tooth is attached to the alveolus by a vascular layer of connective tissue, the alveolar periosteum (Periosteum alveolare), which constitutes the periosteum at once of tooth and alveolus.

The blood-supply to the pulp is derived from the alveolar or dental branches of the internal artery; the nerve-supply comes from branches of the trigeminus.

The Permanent Teeth

The formula of the permanent teeth of the horse is:

In the mare the canines usually are very small and do not erupt, reducing the number to 36 or 38.^[81]

Incisor Teeth.—These are twelve in number. The six in each jaw are placed close together, so that their labial edges form almost a semicircle. They have the peculiarity (not found in existing mammals other than the equidæ) of presenting, instead of the simple cap of enamel on the crown, a deep invagination, the infundibulum, which becomes partly filled up with cement. Hence as the tooth wears the table surface has a central ring of enamel surrounding this cavity in addition to the peripheral enamel. The cavity becomes darkened by deposits from the food, and is commonly termed the “cup” or “mark.” Each tooth is curved so that the labial surface is convex and the roots converge. The average length of the incisors at five or six years of age is about two and a half to three inches (ca. 7 cm.). They taper regularly from crown to root, without any constriction, and in such a manner that in young horses the exposed crown is broad transversely; toward the middle, the two diameters of a cross-section are about equal; near the root the antero-posterior diameter is considerably greater than the transverse.

This fact is of value in the determination of age by the teeth, since the table surface at different ages represents a series of such cross-sections. As the exposed crown wears down the embedded part (reserve crown) pushes up out of the alveolus, so that the tables of the first and second lower incisors are at first oval, with the long diameter transverse; later—at about fourteen years usually for the first lower incisors—the tables are triangular, with the base at the labial edge. At the same time the infundibulum or cup becomes smaller, approaches the lingual border, and finally disappears; it remains longer on the upper incisors, as it is deeper in them. Another marked feature in old age is the progressive approach to a horizontal direction as seen in profile; at the same time the teeth become parallel and finally convergent.

Canine Teeth.—These are four in number in the male; in the mare they are usually absent or rudimentary.^[82] They interrupt the interdental space, dividing it into two unequal parts. The upper canine is situated at the junction of the premaxilla and the maxilla; the lower canine is placed nearer the corner incisor. The canines are simple teeth, smaller than the incisors, and are curved with the concavity directed backward. The crown is compressed, convex, and smooth externally; concave with a median ridge internally; its edges are sharp, and the apex is pointed in the unworn tooth. The root is round and the pulp cavity is large, persisting to advanced age.

Cheek Teeth (Premolars and Molars).^[83]—The constant number of these is twenty-four—twelve in each jaw. Quite commonly, however, the number is increased by the presence in the upper jaw of the so-called wolf tooth. This tooth is usually situated just in front of the first well-developed tooth; it is a much reduced vestige, not often more than one-half or three-fourths of an inch (ca. 1 to 2 cm.) in length. (It is interesting as being the remnant of a tooth which was well developed in the Eocene ancestors of the horse.) It may erupt during the first six months, and is often shed about the same time as the milk-tooth behind it, but may remain indefinitely. The occurrence of a similar tooth in the lower jaw—which rarely erupts—increases the dental formula to 44, which is considered the typical number for mammals. They may be regarded as belonging to the permanent set, having no predecessors. The cheek teeth are very large, prismatic in form, and quadrilateral in cross-section, except the first and last of the series, which are triangular. The crown is remarkably long, most of it being embedded in the bone or situated in the maxillary sinus in the young horse. As the exposed part wears down the embedded part pushes up to replace it, so that a functional crown of about four-fifths of an inch (ca. 2 cm.) is maintained. The root begins to grow at four or five years of age, and is complete at twelve to fourteen, although the deposition of cement may continue indefinitely.

The upper or maxillary teeth are embedded in the alveolar processes of the maxilla. The exposed parts of the crowns are normally in close contact, forming a continuous row which is slightly curved, with the convexity toward the cheek. The embedded parts diverge in the manner shown in the annexed figures (Figs. 251, 253). Thus the long axis of the first is directed upward and a little forward, that of the second is almost vertical, while in the remainder it is curved backward in an increasing degree. The average length at six years of age is about three to three and a half inches (ca. 7 to 9 cm.). The buccal (or outer) surface presents a central ridge running lengthwise, and separating two grooves; the first tooth has, in addition, a less prominent ridge in front of the primary one. The lingual (or inner) surface is marked by a wide, rounded ridge, the accessory pillar or column, which separates two very shallow grooves. The masticatory or table surface presents two infundibula, an anterior and a posterior. It slopes obliquely downward and outward, so that the outer edge is prominent and sharp. The first and last teeth have three roots, the remainder four or three.

The position of the embedded crowns and roots of the last four varies at different ages and in different subjects. Two factors in this variation may be noted. All these teeth are developed in the maxillary sinus close to the orbital fossa. As growth proceeds the teeth move forward, so that commonly only the last three, but sometimes also the third, remain (except as to their free crowns) in the sinus. The second cause of variation is the fact that the anterior limit of the maxillary sinus may be at the extremity of the facial ridge, or more than an inch beyond it. In the latter case the third tooth projects into the sinus.^[84]

The structure is quite complex. Two infundibula run vertically through the entire length of the crown; these become filled with cement. There are five main divisions of the pulp-cavity and five enamel folds, four of which are arranged symmetrically, while the fifth is an outgrowth from the inner side of the antero-internal fold. On the exposed crown of the unworn tooth the enamel folds form rounded ridges covered with a thin layer of cement. After the tooth comes into wear the enamel on the masticatory surface stands out in the form of sharp prominent ridges. Progressive cementation of the periphery of the tooth takes place, thus leveling up the irregularities of surface to a considerable extent.

The lower or mandibular cheek teeth are implanted in the rami of the mandible, forming two straight rows which diverge behind. The space between the rows is considerably less than that separating the upper teeth, especially in the middle of the series. The length of the lower teeth is about the same as that of the upper set. Their direction is also similar, but the embedded portions diverge even more, with the exception of the first and second. The long axis of the first is vertical; the remainder project downward and backward in a gradually increasing obliquity. The buccal (outer) surface has a longitudinal furrow; the last molar has a secondary, shallower furrow in addition. The lingual (inner) surface is uneven, but the grooves are not regular; there are usually three on the first and last tooth. The masticatory or table surface is oblique, sloping upward and inward in correspondence with the opposing tooth; thus the inner edge is prominent. The first five have two roots, while the sixth commonly has three. The width of the lower molars is a little more than half that of the upper. There are two infundibula, which are open along the inner face of the tooth until closed by cement. The pulp-cavity has two principal divisions, and four or three secondary diverticula. The enamel folds correspond, forming a pattern even more complicated than on the upper teeth.

The deciduous, temporary, or “milk” teeth (Dentes decidui) are smaller and fewer than those of the permanent set. The formula is:

The deciduous incisors are much smaller than the permanent ones. They have a distinct neck at the junction of the crown and root. The crown is short, white in color, and its labial surface is smooth. The infundibulum is shallow. The root is flattened; it undergoes absorption as the permanent tooth develops behind it.

The deciduous canines are quite vestigial. They occur in both sexes as slender spiculæ about a quarter of an inch in length, but do not erupt. The lower one develops close to the corner incisor. They are not usually included in the formula, as they are never functional.

The deciduous cheek teeth differ from the permanent set chiefly in that they have much shorter crowns than the latter. The roots form early, so that a distinct neck occurs.

The subjoined table indicates the average periods of the eruption of the teeth:

(The periods given for P 3 and 4 refer to the upper teeth; the lower ones may erupt about six months earlier.)

THE SALIVARY GLANDS

This term is usually restricted to the three pairs of large glands situated on the sides of the face and the adjacent part of the neck—the parotid, submaxillary, and sublingual. Their ducts open into the mouth.

The parotid gland (Glandula parotis) (Fig. 172)—so named from its proximity to the ear—is the largest of the salivary glands in the horse. It is situated chiefly in the space between the ramus of the mandible and the wing of the atlas. It is somewhat triangular in shape, the apex partially embracing the base of the external ear. Its length is about eight to ten inches (ca. 20 to 25 cm.), and its average thickness nearly an inch (ca. 2 cm.). Its average weight is about seven ounces (ca. 200 to 225 g.).

It presents for description two surfaces, two borders, a base, and an apex. The external (or superficial) surface is covered by the parotid fascia, the panniculus, and the parotido-auricularis muscle. It is crossed obliquely by the jugular vein, which is embedded in the gland tissue to a varying extent. It is also related to the great auricular vein, the cervical branch of the facial nerve, and branches of the second cervical nerve. The internal (or deep) surface is very uneven, and has numerous important relations. Some of these are: the guttural pouch, and the great cornu of the hyoid bone; the masseter, stylo-maxillaris, digastricus, and occipito-hyoideus muscles; the tendons of the mastoido-humeralis and sterno-cephalicus (which separate the parotid from the underlying submaxillary gland); the external carotid artery and some of its branches; the facial nerve; the pharyngeal lymph glands. The anterior (or facial) border is closely attached to the ramus of the mandible and the masseter muscle; it overlaps the latter to a varying extent. (In some cases there is a well-marked triangular facial process, which covers the temporo-mandibular joint, the facial nerve, and the transverse facial vessels.) The posterior (or cervical) border is somewhat concave, and is loosely attached to the underlying muscles. The base or ventral border is related to the external maxillary vein. The apex is attached to the base of the external ear, which it partially embraces.

The gland has a yellowish-gray color and is distinctly lobulated. It is inclosed in a capsule formed by the parotid fascia. The parotid duct (Ductus parotideus Stenonis) is formed at the lower part of the gland, near the facial edge, by the union of three or four radicles. It leaves the gland about an inch (ca. 2 to 3 cm.) above the external maxillary vein, crosses the tendon of the sterno-cephalicus, and gains the inner face of the pterygoideus internus. It then runs forward in the submaxillary space below the external maxillary vein and winds around the lower border of the mandible behind the vein, passes upward between the vein and the masseter muscle for about two inches (ca. 5 cm.), turns forward underneath the facial vessels, and perforates the cheek obliquely opposite the third upper cheek tooth. Before piercing the cheek it is somewhat dilated, but its termination is small, and is surrounded by a circular mucous fold (Papilla salivalis).

Blood-supply.—Branches of the carotid and maxillary arteries.

Nerve-supply.—Trigeminal, facial, and sympathetic nerves.

The submaxillary gland (Glandula submaxillaris) is much smaller than the parotid. It is long, narrow, and curved, the dorsal edge being concave. It extends from the fossa below the wing of the atlas to the body of the hyoid bone, so that it is covered partly by the parotid gland, partly by the lower jaw. Its length is about eight to ten inches (ca. 20 to 25 cm.), its width an inch to an inch and a half (ca. 2.5 to 3 cm.), and its thickness about half an inch (ca. 1 cm.). It weighs about one and a half to two ounces (ca. 45 to 60 g.). It is often divisible into two parts.

It presents for description two surfaces, two borders, and two extremities. The external surface is covered by the parotid gland, the stylo-maxillaris, digastricus, and pterygoideus internus muscles. The tendon of the sterno-cephalicus crosses this surface, and is a useful guide in separating the parotid gland from it. The internal surface is related chiefly to the flexor muscles of the head; the guttural pouch; the larynx; the division of the carotid artery; and the tenth, eleventh, and sympathetic nerves. The superior border is concave and thin. It is related to the guttural pouch and the duct of the gland. The inferior border is convex and thicker. It is related to the thyroid gland and the external maxillary vein. The posterior extremity is loosely attached in the fossa atlantis. The anterior extremity lies at the side of the root of the tongue, and is crossed externally by the external maxillary artery.

The submaxillary duct (Ductus submaxillaris Whartoni) is formed by the union of small radicles which emerge along the concave edge. It runs forward along this border, and, after leaving the anterior extremity, crosses the intermediate tendon of the digastricus, passes between the hyo-glossus and mylo-hyoideus, and gains the inner surface of the sublingual gland. Its terminal part lies on the body of the mandible, under the mucous membrane, which it pierces opposite the canine tooth. The orifice is at the end of a flattened papilla (Caruncula sublingualis).

Blood-supply.—Occipital, external carotid, and external maxillary arteries.

Nerve-supply.—Chorda tympani and sympathetic nerves.

The sublingual gland (Glandula sublingualis) is situated beneath the mucous membrane of the mouth, between the body of the tongue and the ramus of the mandible. It extends from the symphysis to the third or fourth lower cheek tooth. Its length is about five or six inches (ca. 12 to 15 cm.) and its weight about half an ounce (ca. 15 to 16 g.).

It is flattened laterally, and has a thin upper border which causes an elevation of the mucous membrane of the floor of the mouth, termed the sublingual crest (Plica sublingualis). The external surface is related to the mylo-hyoideus muscle, and the internal surface to the genio-glossus and stylo-glossus, the submaxillary duct, and branches of the lingual nerve. The inferior border is related to the genio-hyoid muscle.

The sublingual ducts (Ductus sublinguales minores), about thirty in number, are small, short, and twisted; they open on small papillæ on the sublingual crest.

Blood-supply.—Sublingual artery.

Nerve-supply.—Trigeminal and sympathetic nerves.

THE PHARYNX

The pharynx is a musculo-membranous sac which belongs to the digestive and respiratory tracts in common. It is funnel-shaped, the base joining the mouth and nasal cavity, while the apex is continued by the œsophagus. Its long axis is directed obliquely downward and backward, and has a length of about six inches (ca. 15 cm.).

The pharynx is attached by its muscles to the palate, pterygoid, and hyoid bones, and to the cricoid and thyroid cartilages of the larynx.

Its principal relations are: dorsally, the base of the cranium and the guttural pouches; ventrally, the larynx; laterally, the internal pterygoid muscle, the great cornu of the hyoid bone, the internal and external maxillary arteries, the glosso-pharyngeal, superior laryngeal, and hypoglossal nerves, the submaxillary salivary gland, and the pharyngeal lymph glands.

It presents seven openings. The posterior nares (Choanæ) communicate dorsally with the nasal chambers. The pharyngeal orifices (Ostia pharyngea) of the two Eustachian tubes are situated on the lateral wall behind the nares and a little below the level of the inferior nasal meatus. They are slit-like openings, slightly oblique downward and backward, and are a little more than an inch (ca. 3 cm.) in length. They are bounded internally by a valvular flap formed by the expanded extremity of the cartilaginous Eustachian tube. The isthmus faucium is situated below and in front. It is closed by the soft palate except during swallowing. The laryngeal orifice (Aditus laryngis) occupies the greater part of the ventral wall or floor of the pharynx. Behind this is the œsophageal opening.

The wall of the pharynx comprises from without inward: the muscles, the pharyngeal aponeurosis, and the mucous membrane.

The muscles (Figs. 243, 244) are covered by the pharyngeal fascia, which is attached to the base of the skull, the great cornu of the hyoid bone, and the thyroid cartilage of the larynx. They are as follows:

1. The stylo-pharyngeus arises from the inner surface of the dorsal third of the great cornu of the hyoid bone, passes downward and inward, and enters the wall of the pharynx by passing between the pterygo-pharyngeus and palato-pharyngeus. Its fibers radiate, many passing forward, others inward beneath the hyo-pharyngeus. It raises and dilates the pharynx to receive the bolus in swallowing.