Part 118

=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.

FIG. 246.—LOWER INCISOR AND CANINE TEETH OF HORSE, FIVE YEARS OLD. ]

FIG. 247.—UPPER INCISOR AND CANINE TEETH OF HORSE, FIVE YEARS OLD.

I^1, I^2, I^3, Incisors; C, canine. ]

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. 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.

FIG. 248.—LONGITUDINAL SECTION OF PERMANENT INCISOR TOOTH OF HORSE, ABOUT NATURAL SIZE. ]

FIG. 249.—INCISOR TOOTH OF HORSE, LINGUAL ASPECT. (After Ellenberger-Baum, Anat. für Künstler.) ]

FIG. 250.—CANINE AND INCISOR TEETH OF HORSE.

The bone has been removed to show the embedded parts of the teeth. (After Ellenberger-Baum, Anat. für Künstler.) ]

=Cheek Teeth= (Premolars and Molars).—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.

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.

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.

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: