THE HORSE UNDER DOMESTICATION ar) enamel edges almost as hard as glass, supported on one side by dentine and on the other by cement. These materials being somewhat softer than enamel, wear down slightly in advance, leaving the enamel edges sufficiently exposed to give a perfect self-sharpening, grinding surface, but at the same time not projecting so far beyond the general plane as to be in danger of chipping off. When we compare such a tooth as this with a perfectly simple one, composed of dentine and having a_ short, either cone-shaped or flattened crown, covered merely with a smooth shell of enamel without cement, it is difficult at first sight to see any relation or similarity between them, or to realize that the highly specialized organ is simply a development of the primitive one. But let us by way of illustration imagine for a moment a tooth with a crown in form somewhat like a pillar or post, composed of dentine with two deep depressions in the top, the dentine being completely covered with enamel, and a thick deposit of cement on the surface of the enamel. When the cement and enamel are worn through on the grinding surface we would have the exposed edge of an enamel cylinder filled with dentine, reénforced with cement on the outer surface, and two enamel rings, or rather pockets with the edges exposed, 1mbedded in the dentine. These pockets or depressions would be lined on their inner surfaces, or completely filled, as the case might be, with a deposit of cement. This would be a construction similar to that we have already seen in the incisor, except that we have two instead of the single cul-de-sac. Now let us imagine the enamel walls of these culs-de-sac, and also the enamel cylinder, deeply crenated or folded and doubled, in some places the latter coming almost in contact with the former, thus increasing greatly the exposed enamel edges, and we will have the design of the grinding surface of an upper deciduous molar after it is somewhat worn (Fig. 33A). And here it must be borne in cement natural cavity Fig. 33. Wearing surface of upper grinding teeth. Natural size A.
Wornsurface of deciduous molar (d.
m.*) of colt about 6 months old.
e, exposed enamel edges; 7, natural cavity in cement Unworn surface of deciduous molar(d.m.+)ofcoltabout3 months before birth, showing only enamel. d, cul-de-sac to be filled later with cement; e, apex of folded edge of enamel C.Premolar(p.m.?)ofhorse8or 9 years old
[] enamel dentine cement B natural cavity oO AMERICAN MUSEUM GUIDE LEAFLETS mind that the horse’s teeth are not fully functional until the folded edges of enamel on the grinding surface are worn through to the dentine. , Figure 336 shows the deciduous upper molar (d. m. *) of a colt about three months before birth. The enamel of the culs-de-sace is continuous with that of the exterior, the folded edges on the grinding surface being unworn. It is as yet entirely without cement so that we have an unobstructed view of the enamel. When the colt is six months old, however, the culs-de-sac will be practically filled with cement and also the deep folds of the exterior (Fig. 334), the deposit of cement beginning about a month before birth.
We have seen that the permanent incisors are much more highly specialized than are the deciduous ones (Page 48). This is quite as noticeable in the grinding teeth. Observe the wonderful development of enamel edges in figure 33C. This is a premolar of a horse eight or nine years old. A careful measurement of the exposed enamel in this tooth reveals the interesting fact that if these edges were straightened out in a continuous line, it would be fourteen inches in length. In a tooth of this size on the simple “cylinder” plan, it is obvious we would have only about four inches of enamel, hence we can willingly concede nature’s wisdom in this comparatively ‘‘modern improvement.”
This enamel pattern however, presents a more simple aspect as the tooth wears down toward the base of the crown, so that in old age when the grinding surface is near the root, there is less enamel, and the self-sharpening surface is much less perfect. The antero- Fig. 34. Upper grinder (p. m. +) of horse 10 or 11 vears old cut in 5 sections; showing pattern of enamelatvariousages h.Branchesofthepulpcavity C, E,-k.Pulpcavity
THE HORSE UNDER DOMESTICATION a7 posterior diameter of the crown is also reduced toward its base. Figure 34 shows an upper right grinder (p. m. *) of a horse ten or eleven years old. The tooth is cut in five sections, and reveals on each cut surface, the pattern of enamel which will be presented on the naturally worn grinding surface at various ages.
Section A is a diagram of the natural grinding surface as it appeared in life, and shows a very complicated pattern of enamel, and large culs-desac almost completely filled with cement.
Section C which is cut one and one-fourth inches farther along the crown, shows great simplification of enamel pattern, and the culs-de-sac are much smaller. The five branches of the pulp cavity (h) are open, but would have been filled with new dentine before the natural- grinding surface reached this point which takes place when the horse is approximately twenty-five years old.
Section LE. The lower border of enamel which marks the base of the crown where this section is cut, is somewhat irregular, so that the enamel line is incomplete. This explains the three spaces which can be seen in the external enamel. The culs-de-sac are entirely eliminated and the enamel edges are reduced to a minimum. The anteroposterior diameter is also greatly reduced.
The branches of the pulp cavity are united at this point (h), where the roots begin to diverge.
ete Fajiver’ “pre- This tooth, p. m.*, being the youngest tooth molar (p. m. =) of horse8 1 the grinding set, would not be worn down to or 9 years old the base of the crown until the horse reached thirty-five or more years, an age very rarely attained. Sections of grinding teeth on exhibition, particularly specimen No. 95, will be of interest as illustrating structure and wear. The lower molars in their construction are quite similar to the uppers, one of the most striking differences being the absence of the culsde-sac. Much the same result is acquired however, by the very deep folding in of the external enamel, which at various points almost meets toward the center (Fig. 35). The lower molars are also narrower in their transverse measurement.
There are several points which sometimes lead to a little confusion in observing the grinding surface of the molars. For example, at the bottom of the deeper concave surfaces of dentine between the enamel lines, there is a dark brown mark which is sometimes mistaken for a fourth material, but is merely a coloration of the dentine. In the upper molars we generally find two small cavities (Fig. 334 and C), one in each cul-de-sac, which sometimes gives rise to question-
5S AMERICAN MUSEUM GUIDE LEAFLETS ings, and indeed there have been swindlers masquerading as veterinary dentists, who would gladly ‘fill’? your horse’s teeth for a reasonable consideration. These cavities however are perfectly normal, and we have precisely the condition here as in the incisors except that in the molar the cement is very much thicker, and the presence or absence of the ‘avity depends simply upon the thickness of the cement. It will be seen that where the tooth comes in close contact with its next neighbor, the cement is missing, and indeed, sometimes the continuous line of external enamel is broken (Fig. 35). This is due to a slight wear on the side of the tooth. There is a strong tendency to crowd toward each other, thus preventing the forcing of food material between them. The slight vibration when in use, of each tooth in its alveolar socket, affords sufficient wear to keep them perfectly fitted together, and also explains the apparent break in the external enamel wall, which we frequently see. This crowding together, as well as many other features, ‘an best be observed from the side view. On exhibition are a number of skulls having the bone cut away in order to expose to view the full length of the tooth.
The three posterior teeth or molars, incline very considerably to the front, while the other three, the premolars, lean slightly toward them (Fig. 37), so that when pressure is applied on their grinding surfaces they are forced together like stones in the arch of a building. And asthe teeth wear down to a point of smaller diameter, the last molar in extreme old age assumes an almost horizontal position (Fig. 37C). The grinders from a lateral view present a most formidable looking dental battery.
It gives some idea of their development when we find that a full set of premolars J and molars, taken from a fair-sized horse of four or five years, will weigh about four pounds, the premolars being a little larger than the molars, a fact which is quite unusual, asin the great majority of mammals the premolars are very much inferior in size and development, as was also the case in the ancestral horse.!
Wesee from the lateral view as well as elle ci: from the grinding surface, the more highly Fig. 36. Deciduous molar yee (d. m. 3) of colt 11 months Specialized characters of the permanent old. Natural size. a, crown: tooth. Compare the very long crowns of b, base of crown; ¢, root figure 37A with the deciduous molar (Fig. ISee page 15.
——*" 2 . 2 3 ois aS ya f pm jim m=% m " Fig. 37. Dental battery of adult horse A. Skulliyearsold. Permanent teeth allin use it B.Skull8yearsold.Crownsreducedinlengthbywearandrootsgrownlonger.Vestigia] p. m.} present in this individual ; C.Skull39yearsold.Crownsalmostwornaway.Lowermolarsinclineforward.(Thecanines are absent in the female) 39
| 60 AMERICAN MUSEUM GUIDE LEAFLETS 36), with its short crown, clearly defined base line and flaring roots. . Wear and movement of premolars and molars can be studied very readily from ‘the skulls on exhibition, three of which are shown in figure 37.
For the present we will consider only the lower teeth, which will apply in a general way to the upper as well. The growth of the tooth begins at the grinding surface, inside the alveolar cavity, and proceeds toward the root, the tooth constantly moving upward, erupting and beginning to wear off before even the crown is complete. Figure 37A shows the skull of a horse about five years old. The deciduous set has been shed and the permanent teeth are all in use, though the wolf tooth (p. m. 1) is not present in this specimen. The crown of p. m. +, the last tooth in the grinding set to reach the wearing line, is now fully grown, having already lost about half an inch from wear. It now measures three and a half inches, making a total length of four inches to be slowly moved upward and worn away. The roots are just beginning to develop at the base of the crown. The next tooth back of this, m 1, is the first permanent tooth to appear, hence the oldest one in the mouth. The crown is worn down much shorter than that of p. m. 7, and the roots are quite evident, though still incomplete. In figure 37B, showing a specimen about eight years old, the crowns have all been perceptibly reduced by wear, and the roots are all more or less developed, those of m. + being almost completely grown. The oldest skull’ in our exhibit is a specimen from a horse thirtynine years old, figure 37C. The crowns are almost entirely worn away. Some of the roots are very much elongated, while others are thickened by an extreme reénforcement of cement. The molars are very much inclined forward, particularly m.z, which is so worn as to almost separate the two roots. In the upper set, p. m. ~ is represented only by irregularly shaped lumps of cement which surround the last fragments of roots, prolonging to the utmost their very much impaired service.” ERUPTION OF THE TEETH OF THE HORSE HE calcification of all the deciduous teeth and also m. t begins — before birth. When the colt is born, d. 7. t and d. m. = and 3 are very near the surface of the gum, and generally erupt within two or three days; d. m. + is apt to follow during the first month; d. 7. 3, the last deciduous tooth to erupt, makes its appearance as late as the eighth or tenth month. The three deciduous molars seem very large for ‘Since writing, a skull 421% years old has been obtained, Specimen No.
138, on exhibition.
“Specimen No.
132 in wall case shows interesting examples of reenforcement by cement in extreme old age.
Till ‘\ = = s ) <i) v .
- = = ‘ _ = = : :
Fig. 38. Dentition of the colt A.
Colt 2 days old showing deciduous teeth and beginning of m.
B.
Eleven months old. Deciduous teeth somewhat worn and permanent set developing C.Twoyearsold.Deciduousteethverymuchwornandpermanentsetpartlyinuse 61
:
62 AMERICAN MUSEUM GUIDE LEAFLETS the size of the young skull, their antero-posterior measurement being a little greater than that of their successors, the premolars, and occupy the entire space in the skull which will finally accommodate the whole grinding set, so that m.* is crowded far back, beginning its development in the alveolar cavity directly below the eye socket, as shown in figure 38A, a colt two days old. This discrepancy, however, is nicely adjusted. As fast as the increasing size of the skull will permit, the teeth are crowded forward with a horizontal movement, making room at the back for the development of the molars, each one resigning its birth-place below the eye, and moving forward to make room for the next one to fallin line, the operation being about the same in the lower set as in the upper teeth. The incisors also encroach upon the region of the very small d. ¢c. + particularly d. 7. 3, which begins its development well back of the d. ¢. — (Fig. 384A). As the Jaw grows, d. 7. 3 passes forward and assumes its final position in front of the d. c. —.
At eleven months (Fig. 388) the deciduous teeth are all in use and most of them considerably worn. m. ~*~, which is due to erupt within about a month, has increased wonderfully in length and has moved forward, giving up the place under the eye to m. ~, which has already attained considerable size, and has advanced somewhat in its forward movement. Between the roots of d. m.~, which are flattened and widely spread apart, thus making room for the new tooth, the calcification of p.m. is just beginning. 7. 1 is developing, and can be seen in specimen No. 38! far back in the alveolar cavity under the root of d. 7. +, but is not visible in the figure.
In figure 38C, a colt of two years, calcification has begun in all the permanent teeth except 7. >. The crowns of the deciduous molars are nearly worn away, while the roots are being absorbed by the growth of the premolars which have now attained a considerable size. m. +, the first permanent tooth to erupt, has been in wearing about a year and is just beginning to develop roots. m.* has cut through the gum and is very slightly worn. The alveolar cavity below the eye, is now lodging its last young tenant, m. ~, which by the time it is full grown will have moved nearly two inches anterior to its present position. SHEDDING THE TEETH EFORE the deciduous molars are shed the crowns are almost entirely worn away on the one side, while the roots are absorbed by the new teeth on the other, so that there is only a small fragment of tooth really lost. The time of shedding may vary a number of months: ° ° 3 . .
At three years approximately, d. m. 2 and 3 are shed, exposing the grind- 1On exhibition in wall case.