Part 38

=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.

[Illustration:

FIG. 251.—SKULL OF ADULT HORSE SCULPTURED TO EMBEDDED PARTS OF TEETH.

The bone has been removed to show the embedded parts of the teeth _I^1_, _I^2_, _I^3_, Incisors; _C_, canines. The cheek teeth are numbered. ]

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]

[Illustration:

FIG. 252.—CROSS-SECTIONS OF CHEEK TEETH OF HORSE.

Buccal (outer) surfaces face to left. _A_, Upper tooth, _B_, lower tooth; _I_, anterior, _I′_, posterior infundibulum, both almost filled up with cement. ]

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.

[Illustration:

FIG. 253.—SKULL OF TWO-YEAR-OLD COLT, SCULPTURED TO SHOW THE EMBEDDED PARTS OF THE TEETH.

Both permanent and temporary cheek teeth are shown. The upper first premolar (“wolf tooth”) is present, but not visible; the lower one is indicated by the arrow. Temporary chief premolars are numbered _1_, _2_, _3_; permanent premolars and molars are designated by Roman numerals; _Dc._, upper temporary canine; _C_, lower permanent canine, which was not ready to erupt; _Di 2_, _Di 3_, second and third temporary incisors; _I 1_, first permanent incisor, not quite ready to erupt. ]

THE TEMPORARY TEETH

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

( 3 0 3 ) 2 ( Di - Dc - Dp - ) = 24. ( 3 0 3 )

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:

TEETH ERUPTION

A. _Deciduous_: 1st incisor (Di 1) Birth or first week. 2nd „ (Di 2) 4–6 weeks. 3rd „ (Di 3) 6–9 months. Canine (Dc ) 1st premolar (Dp 2) Birth or first two weeks. 2nd „ (Dp 3) „ 3rd „ (Dp 4) „

B. _Permanent_:

1st incisor (I 1) 2½ years. 2nd „ (I 2) 3½ years. 3rd „ (I 3) 4½ years. Canine. (C ) 4–5 years. 1st premolar (or wolf tooth) (P 1) 5–6 months. 2nd „ (P 2) 2½ years. 3rd „ (P 3) 3 years. 4th „ (P 4) 4 years. 1st molar (M 1) 10–12 months. 2nd „ (M 2) 2 years. 3rd „ (M 3) 3½–4 years.

(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.

[Illustration:

FIG. 254.—POSTERIOR PART OF A SAGITTAL SECTION OF HEAD OF HORSE, CUT ABOUT 1 CM. TO THE LEFT OF THE MEDIAN PLANE.

_1_, Posterior nares; _2_, pharyngeal orifice of Eustachian tube; _3_, aditus laryngis; _4_, entrance to œsophagus; _5_, posterior pillar of soft palate; _5′_, junction of _5_ with its fellow over entrance to œsophagus; _6_, epiglottis; _7_, body of thyroid cartilage; _8_, arytenoid cartilage; _9_, _9_, cricoid cartilage; _10_, true vocal cord; _11_, false vocal cord; _12_, lateral ventricle of larynx; _13_, crico-arytenoideus post. s. dorsalis; _14_, œsophagus; _15_, external carotid artery; _16_, hypoglossal nerve; _17_, glosso-pharyngeal nerve; _18_, great cornu of hyoid bone; _19_, Eustachian tube; _20_, body of hyoid bone; _21_, hyoideus transversus; _22_, ridges of hard palate; _22′_, soft palate; _23_, septum between frontal sinuses; _24_, olfactory mucous membrane; _25_, sphenoidal sinus; _26_, basilar part of occipital bone; _26′_, supraoccipital; _27_, body of sphenoid bone; _28_, pituitary body; _29_, chiasma opticum; _30_, corpora quadrigemina; _31_, thalamus; _32_, arachnoid; _33_, odontoid ligament; _34_, posterior auricular muscles. ]

2. The =palato-pharyngeus= arises by means of the aponeurosis of the soft palate from the palate and pterygoid bones. Its fibers pass backward on the lateral wall of the pharynx, and are inserted in part into the upper edge of the thyroid cartilage, in part turn inward to end at the median fibrous raphé. Its action is to shorten the pharynx, and to draw the larynx and œsophagus toward the root of the tongue in swallowing.

3. The =pterygo-pharyngeus= is flat and triangular. It lies on the anterior part of the lateral wall of the pharynx. It arises from the pterygoid bone above the preceding muscle—from which it is not distinctly separated—crosses the levator palati, and is inserted into the median raphé. Its action is similar to the preceding.

4. The =hyo-pharyngeus= may consist of two portions:

(_a_) The =kerato-pharyngeus= is a small and inconstant muscle which arises from the inner surface of the great cornu of the hyoid bone near its lower end. It passes upward and backward, turns inward toward the raphé, and spreads out under the next muscle.

(_b_) The =chondro-pharyngeus=, broad and fleshy, arises from the thyroid cornu of the hyoid bone and by a thin fasciculus from the wing of the thyroid cartilage and ends at the median raphé.

5. The =thyro-pharyngeus= arises from the lateral surface of the wing of the thyroid cartilage. Its fibers pass forward and inward to the median raphé.

6. The =crico-pharyngeus= arises from the cricoid cartilage and ends at the raphé. The fibers are directed upward, forward, and inward; they blend behind with the longitudinal fibers of the œsophagus.

The last three muscles are constrictors of the pharynx.

The =pharyngeal aponeurosis= is attached to the base of the cranium. It is well developed on the inner face of the palato-pharyngeus muscle and forms a median raphé (Raphé pharyngis) dorsally, which is wide in its posterior part.

The =mucous membrane= of the pharynx is continuous with that of the several cavities which open into it. It is thin and closely adherent to the base of the skull in the vicinity of the posterior nares, where the muscular wall is absent. Behind the Eustachian openings is a median cul-de-sac, the =pharyngeal recess=. The recess is somewhat variable, but is usually about an inch in depth and will admit the end of the finger. In the ass and mule it is much deeper. Here also the muscular wall is absent and the mucous membrane lies against the guttural pouches. From the Eustachian opening a fold of the mucous membrane (Plica salpingo-pharyngea) passes toward but does not reach the laryngeal opening. Below, a horizontal fold, the =posterior pillar of the soft palate= (Arcus pharyngopalatinus), passes along the lateral wall and unites with its fellow over the entrance to the œsophagus. The upper part of the cavity (the naso-pharynx) is lined with a ciliated epithelium, while the lower part (oro-pharynx) has a stratified squamous epithelium. The communication between the two is oval and is bounded by the free edge of the soft palate and its posterior pillars; it is termed the =pharyngeal isthmus=. On either side of the laryngeal opening is a narrow deep depression, the =pyriform sinus= (Recessus piriformis).

The submucous tissue contains numerous mucous glands (Glandulæ pharyngeæ). In the young subject the lymph follicles are numerous and form a collection dorsally and between the Eustachian openings, known as the pharyngeal tonsil.

=Blood-supply.=—External carotid, external maxillary, and thyro-laryngeal arteries.

=Nerve-supply.=—Glosso-pharyngeal, vagus, and sympathetic nerves.

THE ŒSOPHAGUS

The œsophagus is a musculo-membranous tube, about 50 to 60 inches (ca. 125 to 150 cm.) in length, which extends from the pharynx to the stomach. It begins in the median plane above the cricoid cartilage of the larynx. In its course it shows several changes of direction. At the level of the fourth cervical vertebra it inclines to the left side of the trachea, and continues this relation to the level of the third thoracic vertebra. Here it again gains the dorsal surface of the trachea, and passing backward, crosses the left bronchus, being here almost in the median plane. It continues in the mediastinum between the lungs backward, upward, and a little to the left, to reach the hiatus œsophageus of the diaphragm. Passing through this it terminates at once at the cardiac orifice of the stomach, at the level of the fourteenth thoracic vertebra, a little to the left of the median plane, and about four or five inches (ca. 10 to 12 cm.) ventral to the vertebral column.

Viewed with reference to the frontal plane, its course is downward and backward till it enters the thorax and passes upward to gain the dorsal face of the trachea. For a short distance (_i. e._, to the root of the lung) its direction is almost horizontal; behind this it passes somewhat upward to its termination. The =cervical part= of the tube is about four to six inches (10 to 15 cm.) longer than the =thoracic part=, while the so-called abdominal part is about an inch (2 to 3 cm.) long.[85]

[Illustration:

FIG. 255.—CROSS-SECTION OF HEAD OF HORSE.

The section passes through the temporo-mandibular articulation, but is slightly oblique. _1_, Corpus callosum; _2_, lateral ventricle of brain; _3_, caudate nucleus; _4_, internal capsule; _5_, lenticular nucleus; _6_, optic chiasma; _7_, middle cerebral artery; _8_, sphenoidal sinuses; _9_, cavernous sinus; _10_, Eustachian tube, inner lamina; _11_, _11_, guttural pouches; _12_, soft palate; _13_, epiglottis; _14_, hyo-epiglottic muscle; _15_, thyro-hyoid muscle. ]