The relative positions of the various structures that enter into the composition of the incisor tooth, will be better understood by reference to the accompanying wood-cut, which represents one of those members divided down its centre:—a indicates the crusta petrosa, which can be traced to enclose the organ, and to dip down the infundibulum B. 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. b denotes the ivory, which forms the principal and central portion of the tooth, in the middle of which is the darker space marked by the letter d. 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 ivory b, and the crusta petrosa a, is a white line c, 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 petrosa must 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, soon sufficiently 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 the rack, 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.
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.