The relative positions of the various structures that enter into the composition of the incisor tooth, will bebetter understood by reference to the accompanying wood-cut, which represents one of those members divided down its centre:—aindicates the crusta petrosa, which can be traced to enclose the organ, and to dip down the infundibulumB. The crusta petrosa is shown to be thicker at the root of the fang, and at the base of the infundibulum, than at other parts.bdenotes the ivory, which forms the principal and central portion of the tooth, in the middle of which is the darker space marked by the letterd. This last is intended to represent the cavity of the pulp, which, in the young incisor is, as in the diagram, of great length. Between the ivoryb, and the crusta petrosaa, is a white linec, which indicates the position of the enamel, and will be seen to cover the upper surface of the tooth dipping into the infundibulum, but not to extend quite to the root.
14.The uses of the ivory, enamel, and crusta petrosamust be now noticed. The ivory is less dense than the enamel, and harder than the crusta petrosa. On the external surface of the incisor teeth, the crusta petrosa is, by the attrition to which this part must be subjected during the gathering of the food, soonsufficiently removed to expose a portion of the enamel: as the years of the animal increase, the outer coating is almost worn away, and not being reproduced, little of the crusta petrosa will be found on the nippers of very old horses. The ivory, however, is always nearly on a level with the enamel, notwithstanding the greater attrition which the substance forming the principal portion of the table of the tooth must necessarily endure. A sufficient indentation of the ivory, nevertheless, can be observed, to render prominent the ridge of enamel, and to indicate that the last-named material is endowed with the greatest power of resistance. The enamel, in fact, is as hard as flint, and by striking it against a steel, fire can be drawn forth. The three structures, therefore, vary in hardness, and in an opposite direction they contrast to one another in toughness. The ivory is sometimes fractured, but not frequently. I have never seen the crusta petrosa of a living tooth exhibit such an injury; but the enamel is rarely inspected without its being discovered to be more or less in a ragged, chipped, and broken condition, especially at the anterior edge of the table of the incisors.
15.The separate uses of the three component structures, however, is not well shown in the incisors; for as the crusta petrosa is by a natural process removed, and the ivory is not of essential service in cutting the food, it might be supposed that the first was no more than a temporary covering to, and the last only a basement for, the enamel. When the mind, however, is directed to the observation of the molars or double teeth, the properties and uses of all become apparent. The sense of touch in the horse resides in the lips; with those organs he gathers together the food before he grasps it with the incisors. Delicate, however, as the animal’s sense of touch is, the selection of the food is further aided by the sight and smell. The most fragrant, the softest, and the cleanest portions, therefore, are selected; but it would be too much to suppose, that no particle of dirt, sand, or grit, was ever taken into the mouth. That much is necessarily bitten, the incisors of those horses which pasture on sandy soils, afford sufficient proof, as such animals generally exhibit the anterior edges of those teeth considerably jagged or notched. In the best fields, the grass is never free from adherent impurities, and the manger, as well as therack, is not always remarkable in that respect. All, however, that is gathered by the lips, or grasped by the incisors, passes to the molars to be comminuted and mixed with the saliva previous to being swallowed. The molars, in fact, are animal grindstones, and the different degrees of hardness which the three component substances display, by wearing unevenly, always keep the grinding surfaces irregular or sharp. The inequality of the grinding surfaces of the molars enables the horse to reduce the toughest fibre to a pulpy mass; but as many substances little less hard than the tooth itself must frequently be ground up with the food, the molars would, at first sight, appear to be subject to injury, especially as they have ten times the work of the incisors to perform, and the senses of touch, smell, and sight, cannot operate for their protection. Nature, however, has provided against the danger to which they appear to be exposed; for if, notwithstanding the guarding agency of the senses, the incisors are so often injured, the molars, blindly employed, and used when the full power of the horse’s jaw is exerted, certainly needed some provision against fracture. The crusta petrosa gives all the security that could be desired; it encircles these teeth, dips into their fissures, and fills their infundibula, forming no small part of the substance of the molars. The subjoined wood-cut will better explain the manner in which the various substances are arranged.
Fig.1.Fig.2.
Figure1 represents the table of one of the molars taken from the upper jaw.Figure2 the table of a molar extracted from the lower jaw. The difference of size between the two organs is thus well marked, as also the difference of shape. The white line indicates the enamel, and the variegated substance which is encircled by it, denotes the comparative quantity and position of the ivory. The crusta petrosa is represented by the somewhat darker shade situated on the exterior of the enamel. In figure 1, however,the reader will observe two patches enclosed by white lines. These are the representatives of the two infundibula which exist in all of the molars of the upper jaw, but which are not developed in the lower teeth. The similar direction of the lines will enable the reader to connect them with the crusta petrosa, which substance, save in the newly-cut tooth, generally fills up these cavities, although the enamel lining extends almost to the root. The intricacy of the arrangement is, by this diagram, made apparent, and the purpose rendered plain. Without such a provision, the first meal of the animal consumed would probably shatter the instruments of mastication into innumerable atoms; but girded round, and bound up with the tough and resistant crusta petrosa, however hard may be the fibre the animal chews—however much of grit or sand may be contained in it—or however great may be the force by which the molars are pressed together, and made to pass at the same time from side to side—not a particle of the brittle enamel is fractured. Like a thin layer of glass, guarded between two pieces of wood, it performs its office in security; and as the necessity for the provision, towards which I have directed attention, is obvious, so is it the more strange that the thick coating of the crusta petrosa around the molars should by authors have been mistaken for an accumulation of tartar. The mistake was certainly extraordinary, as the crusta petrosa in the horse’s tooth is of physiological importance, and will presently be shown to merit the attention of the pathologist. I am aware, when making this statement, that contradiction is offered to the opinions of many and deservedly esteemed authorities; and while I regret the necessity of differing with such writers, I also lament that my opinion as to the supposed use of the outer membrane of the newly-developed organ, does not coincide with theirs. The nature of this treatise will not allow me to enter into the subject of the development of the teeth; but I may here say, that the outer membrane has been generally stated to secrete the enamel. It is that membrane which, thickening with the growth of the tooth, becomes the crusta petrosa; but with regard to its secretive powers, I do not imagine it is endowed with any function of that description, or that it is in any way concerned in the production of the enamel. My reasons for making such and so bold an assertion are well considered,not hastily advanced. I have before me the left anterior molar, taken from the jaw of a colt which was rising four. A representation of the tooth alluded to is here given.
adenotes the crusta petrosa,bthe outer surface of the enamel, the lines representing the plated or finely grooved aspect, which this part exhibits on its exterior.c, which letter will, on inspection, be discovered in the centre of the darkened space to the right of the lower portion of the enamel, indicates a spot where disease was existing, and to which reference will be hereafter made.ddenotes the roots, which, as will be seen, are (although the toothhad been in active employment for twelve months) still incomplete, and not enveloped by crusta petrosa. The above wood-cut, the accuracy of which is not to be disputed, shows a comparatively large portion of the assumed secretive membrane to be absent. The deficient part of the outer covering, I may here state, was removed by a natural process, as I myself extracted the tooth. Nevertheless where the crusta petrosa is deficient, the laminated external surface of the enamel is exposed in perfect and normal condition. This evidence appears pretty conclusive, for where the secretive substance is absent, the secretion would hardly appear. On the other hand, if the secretion of the enamel were the special function of the crusta petrosa, then wherever the one was found, the other would also be present. Let the reader, however, refer to the wood-cut inserted at page 21. In that diagram, the crusta petrosa is faithfully represented as thickest at the root; but at this very point the enamel is seen to be entirely wanting. The conclusion towards which such facts point, is almost decisive; but, nevertheless, there are many inferences which help to support it. If a longitudinal section of a tooth be made, a very thin white linemay be plainly seen, indicating, probably between the ivory and the enamel, another source for the production of the substance which the crusta petrosa has been said to secrete. Moreover, if a tooth be boiled for some hours, a very little force will enable any person to pull off the enamel from the ivory; and this experiment likewise suggests the intervention of some gelatinous membrane between the two structures. The microscope also confirms the opinion I advance, and shows that, in the old tooth, the membrane is absorbed, for its remains can only be detected in isolated places. I am fully aware that none of the arguments here advanced are, with the exception of the first, at all conclusive. Appearances are deceptive, and results consequent upon artificial processes by no means to be trusted. After all, however, the necessity for such a secretive membrane is by no means imperative for the production of the enamel. Our views on these points may be too circumscribed, since at the extremities of the bones we see cartilage and osseous structure connected, without any such interposition; and no one expects such a matrix for the vitreous table of the cranial parietes. The crusta petrosa may have its uses apart from anyconjectural function of secretion; and, in further proof that it does not secrete, it can, in the young subject, before the tooth has emerged from its primary cavity, be separated with the greatest facility from the enamel. It serves to protect the enamel, but it likewise answers the end of keeping the tooth firm in the jaw: around the neck of the molars it becomes thickened to a great extent; and at the roots, especially of old teeth, it exhibits considerable substance. The horse’s molars are continually being wrenched from side to side while the food is being ground, and unless they were very firmly mortised, the vessels which nurture them would be continually lacerated: this the crusta petrosa, by entirely filling the alveolar cavity, prevents; and as the molars wear down with age, the thickening of the membrane enables the jaw to retain with security the latest portion of the tooth. I have specimens taken from very aged subjects, where the fang having been worn away, the molar consists merely of the roots of teeth embedded in a mass of crusta petrosa.
16.The various parts and peculiarities of form, now require consideration. If the table of an incisortooth be observed, a cavity will be seen in the centre of it: this cavity, called the infundibulum, is of variable depth. In the temporary or milk teeth, it extends only half way down the crown; in the permanent or horse incisors, it is from three quarters of an inch to an inch and a quarter deep, being of greater depth in the teeth of the upper than in those of the lower jaw, and not of the same magnitude in all horses. The infundibula are indentations in the body of the ivory, and they possess two coverings, an external one of crusta petrosa, and internal investment of enamel: the enamel does not materially vary in thickness upon this part, but the crusta petrosa of the infundibula is, in different subjects, of very unequal extent. In some animals it becomes very thick, and in such, the infundibula seem soon to be obliterated, owing to the crusta petrosa filling up the cavity. A good specimen of the early obliteration of the cavity, by the thickness of its investing membrane, is shown by the teeth preserved with the jaw of the horse Leander, which is now in the possession of Mr. W. Field. That animal, though but four years old, had lost the “marks” in the central incisors. A white line,however, shows that the enamel dips deep into the substance of the ivory, and indicates what would have been the form of the infundibula, had not the crusta petrosa filled up the cavities. The infundibula have received special attention from horsemen, and have been thought to indicate correctly the age of the horse; their lining membrane speedily becomes blackened by the chemical action of the food, &c., constituting what are called the “marks” of the mouth; and as the teeth undergo wear, these marks are ultimately lost. The period at which they may disappear, and the appearances which they may put on, are well shown in a plate which Mr. Fores has published, entitled “The Age of the Horse,” and to that plate I therefore refer the reader; but, while doing so, I must caution him not to depend too much upon a sign, which is only true as a general rule; for as the infundibula in all animals are not of the same depth, or supposing them to be of one depth, the crusta petrosa is seldom of the like thickness in any two horses, and the teeth do not wear uniformly in different subjects, of course the indications they exhibit cannot be absolutely relied upon. Those indications, however, ought to be known, andshould never be disregarded. Some animals of seventeen years of age, and even in rare instances of twenty-eight years old, will retain the marks in two or more of the incisors; nevertheless, as a general rule, these signs are worthy of attention, and, taken in conjunction with the illustrations presented in this work, afford all that can be obtained to confirm the judgment.
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, answeringto 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 toothis 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 thesedevelopmentsbeing 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 ofthe 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 thesound 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 hasnot 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 confinedthe 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 pulphas 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 thehorse’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 proportionedto 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. Thetemporaryteeth 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 Ihave 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 tooththe 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 creaturebecomes 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 incisorare contrasted.Figure1 is the horse,Figure2 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.
However, while the milk tooth is retained in the mouth the neck is not particularly well shown, because the gum encircling the crown in a great measure conceals that part. But the form of the outer surface of the crown can hardly be mistaken. In thefoal’s tooth it more or less approaches to a semi-circle, from which the permanent teeth greatly differ, as will be seen by comparing the representations of the two as given in the plates. The outer surface likewise is characterized by peculiar indentations: a number of small channels run along it, taking a course from the neck towards the margin of the table, and giving an irregular fluted appearance to the enamel. There will be seen, on the crown ofFigure2, lines intended to convey an idea of the situation and direction of these little grooves. The table is always oblong, but the infundibula are generally absent after the second year, and when they are present, the cavities are narrow, and obviously of little depth. These numerous signs enable the milk tooth to be recognized with facility, at a single glance, for it is seldom that one of the various indications is absent; and after all have been impressed by observation, the milk tooth can hardly be mistaken.
26.The permanent incisors are indicated by their greater size, a circumstance which the foregoing wood-cut does not exaggerate. The gum is muchfurther retracted, and this gives to these organs an irregular oblong figure, which is very different from the semi-circular shape of the milk teeth. The incisors, however, are not of equal length in all horses, and in some the gum may be so prominent as to give the permanent somewhat the appearance of a temporary incisor; other indications, however, even then, enable the different nature of each to be with certainty distinguished. The enamel is only partially exposed in the horse tooth; the shaded portion ofFigure1, in the preceding wood-cut, denotes the extent to which it is generally covered during the period of dentition; and when the teeth first appear, the enamel is wholly concealed by a covering of crusta petrosa. That covering is never entirely removed—a small portion of it, even in the oldest mouths, is seen near to the gums. It looks like an accumulation of tartar, and is most conspicuous in the tooth that has been newly cut. The presence or absence of this substance, therefore, should be noted, and moreover, the enamel surface should be further inspected. The numerous channels that give so peculiar a fluted appearance to the milk tooth, are not to be seen upon that of a permanent description;but in the stead are to be detected only one or two broad grooves, which extend the entire length of the crown.Figure1, at page 48, indicates the shape and direction of the grooves upon the surface of the permanent tooth, though probably a better idea of them is to be derived from an inspection of the coloured engravings, in which they are more prominently exhibited. By means of such guides, the opinion is rendered positive, and the inspection of the table is hardly needed to confirm it; but the table affords corroborative evidence, which may be of service. The table is broader, and the infundibulum is deeper. In this latter cavity, food accumulates in the permanent, but rarely in the temporary incisor. The gum may so contract as to give to the tooth some appearance of possessing a neck, but in reality no such part is developed in the horse incisors, as will be seen by referring to the previous cut, which may, with profit, be compared with the coloured engravings, which depict these members as they are exhibited in the mouth of the living animal.
27.The temporary cannot be well distinguished from the permanent molars, while the horse is alive.It is true that the table of the first is somewhat narrower than, though of equal length, with that of the last; but the difference is not so marked as to justify an opinion being pronounced upon the imperfect inspection which, under the most favourable circumstances, can be made of these organs. The number of the molars may, however, be counted, and from this the age of the animal deduced with correctness. The operation can be at all times performed, and the fact ascertained. When, however, removed from the jaw, the character of the molar is easily recognized, for the difference in the length and shape of the fang renders it then impossible to confound the temporary with the permanent tooth.
28.The various points which denote youthmust now be noticed. The remarks on this subject, however, must be confined to the mouth, to which this treatise is devoted. In the foal, the membrane of the mouth is of a delicate pink colour, suggestive of its high vascularity. The gums are prominent, for as the permanent are cut posteriorly to the milk incisors, and the germs of the horse teeth are developed even at the time of birth, the anterior part of the jaw ispushed forward to make room for their growth. This gives the little animal somewhat the appearance of being parrot-mouthed, as will be observed by referring to the coloured plate of nine months. Then the palate is low in its position, and the bars appear almost on a level with the tables of the upper teeth, and may even descend beyond them. The lower jaw, also, if felt, will be found to be quite round; and the face, if observed at that part under which the molar teeth are situated, will seem full, or the cheek will look a little blown out. These last signs, more or less, remain till the process of dentition is completed.
29.The indications of ageare no less deserving of notice. The edges of the lower jaw become sharp, and ultimately retracted; the cheeks grow flat; the membrane of the mouth loses its vascularity, and puts on a dull yellowish colour; the gums look hard, and the front of the jaw becomes narrower, while at the same time it inclines more backward, causing the teeth to project horizontally. The inclination which age gives to the teeth, will be easily perceived by comparing the coloured engravings with each other; and the peculiarity of the upper and lowerincisors not being perceptible at the same time in the mouth of a thirty-years old, entirely springs from the acuteness of the angle which they form at that period. The bars, as years accumulate, also change their positions, being drawn upwards. From the space between the corner incisors and the first molars, the tongue on either side protrudes, as if the narrowed cavity left not sufficient room for the organ to repose in. The extent to which the tongue will project may be conjectured, by regarding the representation of it given in the coloured plate of the thirty-year old mouth. At that age, the protrusion is too strongly marked to escape the observation of any one whose attention has once been directed to the circumstance. Accompanying the inability to retain the tongue within the jaws, is a constant flow of saliva, which continually falls from the mouth of the old horse when the lips be held apart. The latter fact, the author, when he made the drawings for the present work, had ample experience of, as in some instances it occasioned serious delay.
30.The incisor teeth, in old age, have their characteristic signs. Mostly they appear unnaturallylong, though in rare instances they are worn down almost to the gums. When long, they will generally show interstices dividing them, such interstices, however, not being free, but occupied by a compact mass of foreign matter, derived from the food, and having a black colour. The reader will, by turning to the coloured plates of twenty and thirty years, see these accumulations indicated. When the incisors are very short, they are arranged in a different order to those of the young animal. The tables of the teeth of the young horse, form almost a semi-circle; in the adult animal, they gradually assume a crescentic order, and as age progresses, they ultimately range, more or less, in a straight line. The teeth also look narrower as the fangs descend with the wear of the organs, and in colour they slightly change. The enamel loses much of its semi-opaque and characteristic appearance, assuming a more dull aspect, and a yellowish tint. The crusta petrosa is to be seen only near to the gums, and within the grooves, which often are deep and very well defined, and within which the remains of the once-investing membrane becomes almost black. Then again, the tables themselves change their shape as the years increase. At first, these surfaces are ofan oblong and somewhat oval figure; by degrees, however, they become angular, and ultimately more or less square. The continental authors have been very minute, and not a little profuse upon the changes of form which the tables undergo, and attribute the alteration to the gradual wear of the teeth. The correctness of this view they endeavour to establish by sawing a tooth through at various places, and attempting to show, that the forms of the surfaces exposed by this process correspond to the shapes assumed by the table during the progress of age. Their deductions can, in the study, be made to appear true, but in the stable will not bear the absolute application to which these writers would put them. By means of this test, Pessina asserted he would tell the age of a horse, accurately, up to the twenty-second year. The Girards acknowledge the tables will not guide them so far, but nevertheless are willing to apply them during the first seventeen years of the animal’s life. I leave the reader to form his own opinion of the value of such speculations—for while I confess they are of some worth as guides to the primary study, and helps to the proper understanding of the cause of those changes of figure to be anticipated—I am not satisfied that any rule drawn from them can be depended upon. For that reason I shall hereafter present the reader with figures of the tables taken from authenticated mouths, rather than amuse him with speculations, which, however imposing they may seem to the ignorant, practice soon discovers to be no more than remote and plausible possibilities.
31.Some judges depend upon the tushes as indicative of the age, and taken in conjunction with the other parts they afford corroborative evidence; singly, however, they should not be relied upon, nor should too much weight at any time be placed upon their appearance. In the first place, the tushes are cut irregularly; they may appear in the mouth between the second and third, and they may not pierce the gums so late even as the eighth year; the periods named being of course the extremes. As a general rule, however, they come up between the fourth and fifth year. The custom, however, which Nature so frequently disregards, can afford no positive guide to the judgment, and the wear which may begin thus early or late, should not be too particularly insisted upon. The tushes when they first show areshaped somewhat like a spear head, smooth and rounded on their external surfaces, but grooved on the side which touches the tongue, pointed at their extremities, and sharp at their edges. The inclination of the young tush is oblique, standing forward, and the situation is comparatively near to the incisors. As the animal’s age advances, the tushes retract, the grooves on the inner surfaces wear out, and the space which separates them from the incisors increases: all observations, however, concerning the tushes, must be accepted with much allowance, for these teeth are very irregular. As a general rule, the lower tush becomes blunt, or rounded at its point and edges, and also of considerable length, while the upper is often so diminished as to be worn almost to the level of the gum. In stallions, kept only for service, the lower tush, however, frequently attains a great length, presenting the appearance depicted in the coloured engravings of the mouths at twelve and sixteen years. In those plates, the reader will observe the tushes no longer point in a forward direction, but have a decided inclination outward, and somewhat backward also. Another indication of extreme age is also worthy ofnotice, although, if too absolutely relied upon, it may occasionally mislead. Tartar, not generally found in any quantity upon the other teeth in the horse, accumulates around the tush, and in very old animals almost conceals it. This excessive deposit I have rarely observed in horses of moderate age.
32.The wolf’s teeth or eye teeth aid us in judging of the age; though we are bound to state of these that they can be received only as presumptive evidence. The wolf’s teeth have not been before alluded to, because they are not concerned in mastication, and are no more than the representatives of those organs which form the continuous chain in the mouths of some other animals. These teeth appear to be of no use to the horse. They are little nodules of tooth-like structure, having minute fangs, which are inserted immediately anterior to the first molars of the upper, being rarely seen in the lower jaw, and when present there, always being even yet more diminutive. These wolf’s teeth are generally shed with the first temporary molars, and therefore if they can be seen, it may be assumed that the permanent molarshave not begunto appear. The assumption, however, must not be converted into an assertion; for in some instances the wolf’s teeth are retained, and, in a few heads, will remain after the horse has reached an age far beyond that of colthood: their presence, therefore, rather leads our expectations than confirms our judgment.
33.The mode in which the horse cuts his teethremains to be pointed out. It is generally said, that the tooth, originally growing in its circumscribed cavity, pushes its way through the parts which oppose its entrance into the mouth. So mechanical an idea is characterized by a small amount of physiological knowledge, and it is time the notions which attributed development to mere force were discarded. The tooth is first a pulpy mass, very soft and highly vascular; the superior part, or that part which primarily shows itself above the gum, is the first to become consolidated. Enamel and ivory are there simultaneously deposited, and gradually these extend towards the fang. When a portion of the fang is completed, and the time has arrived for the tooth to be cut, those structures which oppose the egress of the tooth are absorbed; the absorption taking place,not only on the internal surface, against which the young tooth has been supposed to exert its force, but also upon the external parts, upon which it could have no effect. By absorption the barrier is removed and the tooth then comes forward. The process is very beautiful, and no less worthy of admiration is the manner in which the milk teeth are shed. The incisors lose their fangs, and just when the permanent teeth appear, the crown of the temporary ones is removed from the mouth. The removal of the fangs of the milk incisors is the result also of absorption, and the same process effects the removal of the temporary molars, only its operation is not precisely similar; for in these last, not only the fang but the greater portion of the crown is taken away, and little more than the table is cast off when the remnant of what once was a tooth is at length shed. The process of cutting the molar teeth will be more clearly comprehended by referring to the annexed wood-cut, in which the darkened spots indicated by the line running fromadenotes the permanent tooth already through the gum and in the mouth, though still covered and partially concealed by the retained table of the temporary grinderc. The appearance of thetable of the temporary molar when cast off is shown in the figure immediately underneath, to which the lettercis also affixed, and the projecting portions ascending from its edges exhibit the last remnants of what originally was the fang:b bare molars, well up and worn to a level.