Part 3

_Figure_ 1 represents the table of one of the molars taken from the upper jaw. _Figure_ 2 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.

_a_ denotes the crusta petrosa, _b_ the 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. _d_ denotes the roots, which, as will be seen, are (although the tooth had 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 line may 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 any conjectural 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 incisor tooth 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, and should 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, 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.