The Horse's Mouth, Showing the age by the teeth

by Edward Mayhew

17. There are infundibula also in the molar teeth, but not in all of them. Those of the lower jaw have no infundibula, but only grooves or fissures, into which the crusta petrosa dips, and which it fills up. The manner of the arrangement will be seen in the table of one of these teeth represented in figure 2, page 26. In the same place, the wood-cut, figure 1, depicts the grinding surface of one of the superior molars. Every tooth of the last description, possesses two infundibula, which extend almost to the root, and are possessed of the same coverings as the like cavities in the incisors. The crusta petrosa, however, which lines the infundibula of the superior molars, is much thicker than the similar membrane inverting the like parts in the incisors; consequently, absolute cavities, answering to the marks of the nippers, are of comparatively slight extent, soon obliterated, and not generally found in any of these teeth after the horse is six years old; indeed all the marks are often obliterated before the fifth year.

18. To distinguish an upper from a lower molar tooth is not difficult. The presence of the infundibula in the first would alone enable it to be pointed out; but there are other indications that are worthy of a passing notice. The table of the upper molars is much broader, being compared to those of the lower jaw as nine is to five. Then the form of the table is different in each. The table of the lower tooth is indented on either side, the indentation extending downwards along the fang. The table of the upper molars present two narrow prominences, and two broad grooves on the outer side; and also exhibits one broad prominence upon the centre of the inner side—all of these developments being likewise contained along the fang. The slant of the tables also differs. In the upper jaw, the outer edge is the lowest. In the lower jaw, the tables incline in the opposite direction.

19. The teeth of the horse are very firmly implanted in the jaws. The fangs are of a length which they who have only seen the teeth extracted from the mouth of a human being would hardly imagine. The length of fang of course gives the teeth some security; but this is not the only provision made for that end. The teeth radiate towards a common centre, being widely separated at their roots, but approximating at the crowns. They also incline laterally from the perpendicular, so that the force applied to them never acts in a direct line, or is concentrated upon one particular point.

20. The molars of the lower jaw are the active agents of mastication, for motion, during this process, only takes place in that part of the head in which those teeth are placed. The inferior molars are the instruments that grind—the upper molars are simply the surfaces upon which the food is ground. The whole of the molars, however, are never simultaneously employed. The horse can chew but on one side at a time, for as the inferior maxillary bone is considerably narrower than the superior, if the teeth on one side are brought into apposition, those of the opposite side are necessarily separated. Any one who has observed a horse feeding, will have remarked that the animal, during the act, continually abducts and adducts, the lower jaw not causing it to describe a circle, as does the cow, but urging it first to one side and then to the other. It will be seen, therefore, that the whole force of those strong muscles, which close the mouth of the horse, is employed on a portion of the jaw at the same moment, and the power then exerted must obviously be very great. Recognizing this fact, the reason for all that has been pointed out—the shelving tables—the grooved or indented sides—the double inclination, and the great length of the fangs is at once perceived, for the action is a wrenching one; and the greater extent of, and more uneven, surface presented by the molars of the upper jaw, facilitates the comminution, without imposing weight upon the muscular activity. Had the larger teeth been implanted in the inferior maxillary bone, the labour imposed would have been much augmented, and the end attained have been in no way expedited.

21. Provision has been made by Nature to meet the wear to which the horse’s teeth are subjected. Such provision, however, I am of opinion, has been somewhat mis-stated by the majority of writers who have treated of this matter. Many of these authors dwell greatly upon the growth of the teeth of the horse, as though this function were in constant activity during the life of the animal. The arguments by which their views are supported, are primarily drawn from analogy rather than deduced from observation. Because the teeth of certain animals, especially those classed by naturalists as rodentia, and of which the rat and rabbit are familiar types—because the teeth of these creatures are known to grow, and those of the elephant are also proved to increase—the same is assumed of the horse. The inference is certainly of strength, but nevertheless it is far short of proof; a similarity between the animals is not demonstrated, and a sameness cannot be shown. Moreover, in another direction, an evidence is attempted to be made of the consequence which ensues when one of the molar teeth loses its vitality; the tooth that so suffers is rapidly worn down, and the opposing tooth becomes of an unnatural length, projecting far beyond its fellows. The increase of length in such a case has been supposed to go on in the sound organ at the natural rate, and the growth of the diseased tooth only has been thought to be stopped; hence the amount of wear and of growth is attempted to be measured; but on consideration it will be seen, that something more than the mere cessation of increase has to be calculated. The carious tooth had ceased to be a part of the living body, and with its vitality the organ lost its capability of self-preservation. The condition of the part had changed, and of the strength which vitality bestows, all persons must be conscious. To argue from the results produced upon dead matter, and then apply the deductions to living organs, is a course which no physiologist will sanction. The tooth that had ceased to live, might be quickly worn down, for other reasons than that it had ceased to grow. This is so clear, and the inference to which allusion was formerly made so obviously untenable, that I shall not longer detain the reader by dwelling upon either, but proceed to state my own views of the subject. I admit the teeth of the horse do grow, and only doubt if the process has been properly described. When the crown of the tooth first appears in the mouth, the fang is not completed, and the root has not even been developed. The horse is seven years old before all the roots are perfected, and when these are completed, I doubt if the after growth is material. If the jaw of an old horse be examined, the alveolar cavities will be found to be shallow, showing that the loss, consequent upon the wear, was compensated by the tooth being projected into the mouth, and not by any increase of substance. Again, those animals, the incisors of which retain the marks in extreme old age, may show long teeth, but not of that excessive length which growth would suppose; only such as the want of wear would occasion, supposing the increase to cease when the root was perfected. The fang, in fact, is so much tooth in reserve, and as such answers the purpose for which growth was supposed to be necessitated. I have by me, specimens of old teeth, but the measurement of none of them contradicts the opinion I have advanced, there being but a material increase of the crusta petrosa, which at the root blends with the ivory, and cannot be clearly separated from it. The thickening of the crusta petrosa around the root and neck, probably never ceases during the health of the animal; but to this substance alone is confined the imaginary growth of the horse’s teeth. When the roots are perfected, the length of the tooth is completed, and the only after process consists in a gradual deposition of earthy matter, within the body of the member, the bulk of which is defined. In old teeth, the ivory becomes very dense; and he who attempts to cut through an old and young tooth, will be made aware of a contrast. The pulp in the teeth, after the formation of the root, gradually diminishes and ultimately disappears, in consequence of the ossific deposition. In fact, when growth ceases, consolidation has taken place, and rendered its continuance no longer necessary.

22. The cavity of the pulp has been always stated to exist in the tooth of the horse. In the human tooth it occupies the extremity of the fang, being situated within the substance of the ivory; and the double teeth are known to have as many cavities of this kind as there are roots to the part. The same has been assumed to be the case also with the horse, but I cannot say that investigation enables me to corroborate this opinion. I find no cavity, that is no empty space, constantly present in the fangs of the horse’s teeth, and no separate or defined cavities at the roots of the molars. The bone or ivory may be deficient, more or less, towards the centre of the fang; but this space, not yet occupied by the osseous deposit, is filled by a membraneous substance which is continuous with the crusta petrosa of the root, and will ultimately be converted into bone. I cannot separate the tissues forming the pulp, from the crusta petrosa itself; and I assume the two to be continuous. The only difference I can observe is, that the tissue of the pulp is the finest; but as the vessels of the one primarily pierce through and ramify upon the other, I cannot perceive the necessity for their separation. It will be understood, that I am speaking of these parts in a fresh subject, and not alluding to a mascerated and dried specimen, the appearance presented by the last being calculated to mislead. With age, the pulp diminishes; and in a tooth extracted from a very old animal, no positive remains of it will be found—osseous matter filling up the space which the pulp once occupied.

23. Nature provides the horse with two sets of teeth. The first are small, or of a size proportioned to the diminutive stature of the foal, and are called milk teeth—colt’s teeth—or temporary teeth—either of which terms equally well characterizes them. The temporary teeth are twenty-four in number—six incisors in either jaw, and three molars on both sides of the upper and also of the lower jaw. The incisors sometimes begin to appear in the mouth before birth, though generally the gums are unbroken when the foal is dropped; they commence to be shed when the colt is two years and a half old—none remaining in the jaws after the fifth year. The molars, which are the first, second, and third, or the anterior three, begin to be cast off at two years and a half, being entirely removed between the third and fourth year. It used to be asserted that the last temporary molar was shed between the fourth and fifth year of the animal’s life; but while I was connected with the Veterinary College, I inspected several heads and found none that corroborated the doctrine, which, for half a century, had been inculcated at that Institution. My observations were at first violently contested, and opportunities were sought to uphold the received and old manner of teaching. By slow degrees, however, the truth prevailed; and I have now the pleasure of knowing, that what I pointed out to the professors, those gentlemen at present communicate to their pupils. Several parties, however, have been mean enough to pretend they either are ignorant of what I effected, or believe what I accomplished was the work of another person; while I am sorry to add, that a professor has even gone so far as to lay claim to a discovery, which at the time cost me some pains to fully make out, and more to firmly establish. My observations were, in the first instance, made alone, and the only person who eventually assisted me was Mr. I. K. Lord, of Tavistock. That gentleman did afford me much help, and to him I beg publicly to record my sense of obligation. No other individual was in any manner concerned in the investigation, and I regret that I should be necessitated to make such an assertion. The point, however, is now fully established; and it will be seen, that it may be of practical importance. The molars, as truthfully as the incisors, denote the age; and, as corroborators, they should, in every doubtful case, be inspected.

24. To know the temporary from the permanent teeth, or to be able to distinguish one from the other, is very essential; but such knowledge is not so common as may be generally imagined. Dealers have blundered, and many a gentleman having purchased a yearling for a horse, has afterwards been surprised to discover that the animal was losing some of its front teeth. These kind of mistakes are mostly confined to the smaller breeds—with ponies the incident is common enough, but even with horses it has occurred. I may therefore be excused if I enter rather minutely into those points of difference which distinguish the one from the other. The reader must be content to study well this portion of the treatise, for the age is best marked by the number of milk teeth retained in the jaw, and easiest told during those years when these teeth are in the course of being shed.

25. To recognize a milk incisor tooth the reader must bear in mind that it is of a temporary nature, and intended only to exist during the few first years of the animal’s life, or during its colthood; for when all the permanent teeth are fairly up, the creature becomes a horse. The recollection of their temporary character will cause them to be the more easily distinguished: they look less solid and less firm; they are smaller, more white, and if ideas of beauty and sentiment befit such a subject, have a prettier and more innocent appearance. This is perfectly true as a general rule, but nevertheless must not be absolutely applied; for I have seen rare instances in which the milk teeth were, from constitutional or other causes, so discoloured, as altogether to lose their infantine and prepossessing character—the permanent incisors appearing cleanly by the contrast. On account of so singular a circumstance occasionally happening, if for no stronger reason, it is imperative therefore, even at the risk of being tedious, to enter minutely into this part of the subject. In colour these teeth are white, because the coating of crusta petrosa which originally invests them is particularly thin, and soon, for the most part, disappears, exposing the enamel; their crowns are shorter, and the neck is well developed, because their fangs are narrow and of less length. In the annexed wood-cut, for the purpose of making the dissimilarity the more conspicuous, a temporary and a permanent incisor are contrasted. Figure 1 is the horse, Figure 2 the milk tooth. The fang of the milk tooth is properly represented of considerable less diameter than the crown; and just where it arises will be observed a little shading, which indicates a sinking in, or nipping up, at that particular spot: the place so narrowed is the neck, which in the temporary teeth of the colt is not badly developed.

Fig. 1. Fig. 2.