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THE TEETH OF THE HORSE |COLLECTION IN WALL CASE] HE dental apparatus of the horse is one of the most interesting and highly specialized organs we find in nature. During the course of evolution through meny ages, the teeth have gradually grown more and more complex, becoming constantly more perfectly adapted to the cropping and grinding of coarse hard grasses. They are in fact of such great size and importance that the whole skull seems constructed chiefly for their lodgment and operation, the custody of the brain, eyes and ears being, as it were, of secondary consideration; hence the skull must of necessity be very bulky, and in order that the weight may not be unnecessarily great, large air spaces (sinuses) are provided, aggregating between two and three times the amount of space occupied by the brain (specimens 51 and 5, on exhibition in wall case). Indeed the skull of the horse 18 a beautiful design in arches and braces, resulting in a structure of the greatest possible strength and utility with a minimum of weight and material.
The ethmoids, the bony framework of the organ of smell, are also highly developed, with a complicated system of scrolls presenting an immense amount of surface to the air that is drawn into the nostrils (specimens 5, 10 and 51).
STRUCTURE OF THE TEETH VERY simple form of tooth is that commonly found among the reptiles, 2 mere peglike structure composed of dentine with a very much harder covering of enamel on the crown, and a nerve canal in the center. We have selected three incisor teeth (Fig. 30) to show examples of different degrees of specialization. A, the tooth of a young cow, is of comparatively simple form, being somewhat flattened and having a clearly defined line or neck between the crown which is composed of dentine and completely covered with enamel and the dentine root which is without enamel. The neck, or base of the crown, as in most simple teeth, coincides with the line of the gum. In the horse the composition of the teeth is the same except that we have a third element known as cement, which begins to be deposited on the surface some time before the eruption of the young tooth, covering the enamel almost completely. In design it is more complicated. Instead of the plain upper border (Fig. 830A) there is a very deep indentation or folding in of the enamel, somewhat like the inverted finger of a glove, forming an enamel lined cup or cul-de-sac, running far down into the tooth (Figs. 830C and D), becoming more or less completely +S
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~ cee x 2ea¥ -——_— ¥ ae "2 -.we ~ e 2 Ser eenr sa -4* dentine cement natural cavity Fig. 30. Incisor teeth showing different degrees of specialization. Natural size. Permanenttoothofyoungcow.a,crown;b,baseofcrown;c,root Deciduoustoothofcolt4daysold.a,crown;6,baseofcrown;c,incompleteroot; d,bottomofshallowcul-de-sac;e,enamel;/,cementdepositedonenamel Permanenttoothofcolt2yearsold.a,crown;d,bottomofverydeepcul-de-sac;e, enamel; /, cement SideviewofC.(A,B,andC,innerviews) Ll examel dentine cement natural cavity Fig. 31, Incisor teeth of the permanent set (7. +) showing wear and movement. Inner view. Natural size A.
B.
BO Tooth of a colt 2 years old before it has erupted. e, folded edge of enamel, unworn Toothofhorse4yearsold,a,crown;b,baseofcrown;c,incompletedroot;d,bottomof cul-de-sac; e, worn edges of enamel; g, gum line Toothofhorse12yearsold.c,rootfullygrown;f/f,internalandexternalcement;h,nerve foramen; 7, natural cavity in cement Tooth of horse 17 years old. Jj, “‘dental star’’ Tooth of horse 33 years old 49
ae 50 AMERICAN MUSEUM GUIDE LEAFLETS filled, particularly toward the bottom, with a deposit of cement. - It is usually observed that highly specialized organs are more primitive in the young individual than in the adult. This is very noticeable in one specimen (Fig. 308), which represents the deciduous or milk incisor of a colt about four days old. This tooth has just cut through the gum, and does not as yet show any wear from use. . The enamel cup is very shallow as compared with that of the permanent tooth (C and D). It also shows a clearly defined line at the base of the crown, which when the tooth is fully grown, will coincide with the line of the gum as in the teeth of the cow and other simple teeth. The root of B is not as yet fully grown, but presents a large pulp cavity which will be reduced to a narrow nerve canal as the root grows longer, and tapers to a point, leaving only a very small foramen for the passage of the nerve. Figure 30C shows the permanent incisor of a colt two years old, which erupts and replaces its deciduous predecessor at about three and a half years. The cul-de-sac, as mentioned above, is very deep, running down almost the full length of the crown, the root not yet having begun to develop. There is already considerable cement in the cul-de-sac, and a very slight deposit beginning on the external surface about the upper end of the crown. <A series of lower incisor sections and also vertical sections of incisor teeth, exhibited in the wall case, will explain the structure more clearly.
WEAR AND COMPENSATING MOVEMENT OF THE INCISORS HAT is frequently spoken of as “growth” in the horse’s teeth is not growth, but rather movement. It is true that growth does continue in some few of the teeth until the horse is ten or twelve years old, but only in the completion and closing of the roots (see above), the crowns being all fully developed, including grinders as well as incisors, at about five and one-half years. From the time the tooth begins its service it continues to move out of the alveolar cavity, which fills with new bone, this movement counteracting the rather rapid wear of the cutting or grinding surface, so that in extreme old age some of the teeth may be worn down to the very points of the roots. One of their most remarkable specializations is the extremely long crown which instead of being entirely above the gum line, as with the more simple teeth, human for instance, extends far down into the alveolar cavity, as much as 3% inches in the large premolars, as a reservoir of grinding material. Figure 31 shows a series of upper incisors to explain the manner in which ‘he tooth is worn as it proceeds from the alveolar cavity, the wearing surface altering in contour as it progresses along the crown of the tooth which constantly changes in shape toward the root.
THE HORSE UNDER DOMESTICATION 51 A is the permanent tooth of a colt two years old, before it has replaced its deciduous predecessor or has received any wear, so that the external enamel is continuous with that of the cul-de-sac or internal enamel. The cement now lines the cul-de-sac and is beginning to be deposited on the external enamel. As yet the tooth has no root, but presents a wide-open pulp cavity at the base.
B is the tooth of a horse about four years old. It has recently replaced its deciduous predecessor, being in use only about six months. The folded edge of enamel is worn through, exposing the dentine beneath, and separating the enamel of the cul-de-sac from that of the exterior, with the exception of a very slight connection at the left-hand border. Internal as well as external enamel is covered with cement except on the wearing surface. The base of the crown is not clearly defined but can readily be located as it coincides with the lower border of enamel. It will be seen that this point, instead of being at the surface of the gum as in more primitive teeth, is planted about an inch and three-eighths deep, g indicating the gum line.
C, the tooth of a horse about twelve years old, shows a more triangular wearing surface, having been worn down a distance of perhaps three-eighths or one-half inch. The cul-de-sac is greatly reduced in surface diameter, as well as in depth. The root is fully developed and shows the nerve foramen (h) at the point. With the advance of age the wearing surface of the tooth, molar as well as incisor, must necessarily approach the nerve, which would subject it to exposure. This is provided for by the recession of the nerve and a new growth of dentine, reducing the nerve or pulp cavity and sometimes in extreme old age completely filling it.1. As the tooth wears down this new dentine can be detected in the incisor by a slightly lighter shade in color, and is known among horsemen as the ‘‘dental star.’’ It is indicated in Figure 31D and E. In Figure 31D, showing the tooth of a horse about seventeen years old, the wearing surface is still more triangular. The cul-de-sac is almost worn away; the base of the crown is approaching the gum line. Not only the crown as in the younger teeth, but the root as well, is becoming thickly covered with cement, so that the base line of the crown is obliterated, but can readily be located by sectioning. On the more exposed ridges however, particularly the anterior surface, not shown in the sketch, the cement has been worn through in places by friction of the tongue and lips.
E shows the condition in extreme old age, this specimen being the tooth of a horse thirty-three years old. The cul-de-sac has long since 2?
1See specimen No.
132 under ‘‘ Teeth sectioned to show reduction of pulp cavity’’ on exhibition in wall case.
a 52 AMERICAN MUSEUM GUIDE LEAFLETS disappeared and the only enamel remaining is a very narrow strip on the anterior border. The wearing surface has become elliptical, and the “dental star’? has changed from a transverse line to a rounded spot. The deposit of cement has increased to almost an eighth of an inch in thickness, reénforcing the old declining root, as it were, to prolong its service to the very last. 2 The base of the crown has passed beyond the gum line, and at best the tooth could not last many years longer.
It is by the examination of these indications that horsemen ascertain the age, although it is impossible to judge accurately in the more advanced stages. Especial attention is given to the size and shape, presence or absence of the cul-de-sac, known among horsemen as the “mark.” It should be explained however, that the lower incisors which are more easily examined in the living horse, being somewhat smaller and having a shallower cul-de-sac than the uppers just described, will acquire a certain appearance of wear at an earlier age. For instance, if C and D were lower teeth they would have a similar appearance at the ages of eight and twelve years. Considering the incisor set as a whole, it shows other marked changes with the advance of age. To render effective service the teeth must be in close contact; therefore, as each tooth is reduced in diameter it moves slightly toward the median line, reducing the total transverse measurement of the set. The meeting angle of upper and lower teeth becomes much more acute in old age. An exhibit labeled ‘‘Series of Horse Muzzles’’ in the wall case shows the general appearance of the incisor teeth at various ages.
The canine tooth, which is rarely found in the female except in a very abortive form, is of comparatively simple structure and has no apparent function at the present time, but may have been of service as a weapon at a very early stage of evolution. It receives almost no wear, has little or none of the compensating movement of the incisors and is, in fact, of no very great interest.
DESIGNATION OF THE TEETH HE deciduous or milk teeth, which are shed and replaced by permanent ones, are divided into three classes: in front, the deciduous incisors, back of these, the very small deciduous canines, and still farther back, the deciduous molars. In the permanent set the incisors and canines have replaced the deciduous ones, and the deciduous molars have been replaced by the premolars. Still back of the premolars comes a fourth class, the permanent or true molars, which have no deciduous predecessors.
B Fig. 32. Designation of teeth A.Lowerjawofcolt9monthsand12daysold,markedtoshowdesignationofdeciduousteeth B.
Lower jaw of horse 5 years old marked to show designation of permanent teeth In works on comparative anatomy and zoology the teeth are expressed by initial letters and numbers in fractional form; the numerator denotes the number of upper teeth, and the denominator the number of lower teeth of each class, running back from the median line in front, and considering only one side of the skull. For instance the typical mammalian milk dentition is as follows, ail being deciduous teeth: incisors three, canine one, molars four, which would be expressed d. 7. 3, d.c.td.m.+. The permanent dentition would be 7.3, ¢.1, p.m.4,and m. 3 —forty-four teeth in all. The human subject having only thirty-two permanent teeth, would be expressed 7. 2, ¢. T, p. m. 2, and m. 3. The teeth are numbered from the median line, hence when indicating a certain tooth 7. s would be read, lower incisor three; p. m.~ upper premolar four; lower deciduous canine is expressed d. c.—. Figure 32 shows the designation of deciduous and permanent teeth. A is the lower jaw of a colt nine months and twelve days old, in which the deciduous dentition is all in use, and though the eruption of the molars hasnot yet begun, the development of one at least is well under way. In B,a horse about five years old, the permanent teeth are allin place. It must be borne in mind that when a subject has a reduced dentition the teeth are numbered not necessarily as they appear in the jaw, but according to what is believed to be their true relation in the typical dentition, having forty-four teeth. For instance, in the human subject where there are but two premolars 53
4 AMERICAN MUSEUM GUIDE LEAFLETS known among dentists as bicuspids, the first one in the lower jaw.would be expressed p.m. s. The second would be p. m. 7, assuming that p. m. T and p. m. = are the missing teeth. The teeth of specimens on exhibition, Nos. 10 and 36, are labeled in the manner described.
THE WOLF TOOTH OF THE HORSE N the horse we may say that p. m. t (Fig. 37B) is now in the last stage of elimination. Just in front of the upper premolars there is frequently found a very small abortive tooth commonly known as the “wolf tooth.” It does also occur in the lower jaw but is very rare. It is absolutely non-functional, and is in fact detrimental, as it sometimes gives rise to more or less irritation. So that in the horse the first funetional premolar is p. m. ?. The wolf tooth, though interesting, is rather hard to study owing to its irregularity. When speaking of the permanent teeth it is generally designated as p. m. >, which would be correct if it really belonged to the permanent set; this, however, I am inclined to doubt, although it is frequently found long after the shedding period is over. Nevertheless, it seems highly probable that it is a deciduous tooth which having no successor, isnot shed. In this case it would seem that the permanent tooth had been eliminated earlier in the course of evolution than the deciduous one. But until we have more conclusive evidence on this point probably it will be as well to include the wolf tooth among the teeth with which we find it. In figure 324 there is shown the unusual occurrence of the wolf tooth in the lower jaw. In looking over the fossil ancestors it seems this tooth began to show signs of elimination in the late Oligocene Period, which is estimated to be about two million years ago. At this rate we may reasonably hope for some few social reforms before the horse is entirely rid of this rather undesirable appendix . eho PREMOLARS AND MOLARS OF THE HORSE HEN we compare the molar tooth with its most intricate structure, with the incisor, the latter seems perfect simplicity. Without here discussing the evolutionary history of the tooth, it is extremely interesting to consider these special adaptations, which are merely gradual modifications of a simple form brought about without any radical or sudden changes, but slowly developed in an orderly and shall we say, orthodox manner.
The molar of the horse as we find it to-day is a wonderful structure, presenting on its grinding surface a most complicated system of sharp