THE MAMMALIA—MAN AND BEASTS. 



85 



have the appearance of brilliant needles, perpendicular to the surface of the ivory. 

 The ivory and enamel do not form one body, although they are united to;jether very 

 closely, for the enamel can be detached from the ivory without injuring the latter, 

 and reciprocally. But the essential distinction between them consists in the circum- 

 stance that the enamel does not possess gelatine for its base ; for, although it contains 

 some traces of that substance, they are always very minute in quantity. The enamel, 

 on the other hand, is essentially composed of fluate of lime, wiiich contributes its 

 stony character, and imparts a degree of hardness superior to that of any other por- 

 tion of the teeth, and indeed of any animal substance. 



The Cortex, like the Enamel, is deposited from within, outwards; but it cannot 

 be discovered upon teeth possessed of enamel until the latter is entirely formed. 

 M. F. Cuvier is of opinion that, in teeth composed of ivory and cortex alone, it is 

 deposited over the ivory like the enamel. The intimate nature of cortex is absolutely 

 the same as that of ivory, on which account it might with propriety be termed the 

 external ivory. Gelatine forms its principal base, and phosphate of lime is deposited 

 between the meshes of that substance. The cortex is found in layers more or less 

 thick. It is of an extreme thinness on the projecting surfaces of the molars in the 

 Ruminantia, but is much thicker in the hollows found on the summits of their crowns. 

 It is observed, however, to possess a still greater thickness in the crowns of the teeth 

 belonging to Cachalots. In this place it equals the ivory in quantity and thickness ; 

 for the whiter substance, which surrounds the central part of these teeth, is not enamel, 

 as some Naturalists have supposed, but a true external ivory. 



It commonly happens that the cortex contains nothing but gelatine and phosphate 

 of lime. In some cases, however, it contains some colouring matter in addition to 

 these, as may be seen in the teeth of several Ruminantia, and in the incisors of the 

 Beavers, Pacas, Agoutis, Porcupines, and some others. The colour of the anterior 

 part of these teeth depends upon a very delicate layer of true cortex, as I\I. F. Cuvier 

 ascertained by many careful experiments. The colour becomes brown only on that 

 part of the tooth which projects from the gums, while the portion within them is of 

 a dark green. It has been said that this colour is owing to the presence of iron, 

 and that the change which it undergoes from the contact of the air is a true oxi- 

 dation. 



The above details regarding the structure of the dentary capsule, which produces 

 the teeth, as well as the composition and structure of the teeth themselves, have 

 been hitherto demonstrated upon a very small number of Mammaha, and they are 

 applied only by analogy to the remainder. In fact, the teeth of Man, of some Car- 

 nassiers, Rodentia, and Ruminantia, with the Solipeda and the Indian Elephant, 

 have alone been studied in respect to their dentary capsules, and the substances of 

 which the teeth are composed. It is probable that a special investigation of teeth 

 belonging to other IMammalia may lead to the restriction or extension of some of the 

 preceding observations. 



The above remarks explain to a certain extent the manner in which the crown of 

 the tooth is formed. As the dentary bulb is the mould of the crown, and as the 

 matter which it secretes is deposited upon its surface, the crown cannot fail to exhibit 

 the same projections, hollows, and angles — in a word, to have the same identical 

 figure ; but there is nothing in the structure of this bulb which can explain the form 

 of the roots. 



By the term root is commonly understood, that part of the tooth contained within 

 the gums; but it is essential, as has already been explained, to distinguish those in- 

 sertions which differ from the crown neither in structure nor form, from the roots 

 properly so called, which begin from the neck of the tooth, and diminish gradually, 

 until they terminate in a point more or less obtuse, and more or less irregular. The 

 first are not real roots, but are formed merely by the prolongation of the crown 

 within the gums. 



When the time at which the true roots have to be formed has arrived, the ena- 

 mellating membrane ceases to maintain its activity, and even becomes wholly obli- 

 terated. The bulb and the external membrane alone continue to grow and to pro- 

 duce roots, which usually correspond, in number and situation, with the principal 

 tubercles of the crown, and appear to be numerous in proportion to the number of 

 leading branches which the maxillary arteries transmit into the bulb. It seems 

 probable that these vessels and their branches form an inferior prolongation of the 

 bulb, as soon as the crown has been deposited ; or, in other words, that the bulb 

 continues to develop itself under their influence, which is restricted to the points 

 immediately surrounding them. Under this point of view, the roots of the teeth 

 may be regarded as the evanescent crowns of the same teeth, reduced to a rudi- 

 mentary state; for we can easily see how they might be continued, if the vascular 

 system did not become obliterated. In fact, those teeth, where the capsule never 

 ceases to reproduce the crown as fast as it wears away, and which are consequently 

 destitute of true roots, only become such in consequence of the undiminished vitality 

 of their bulb, which continually maintains its vigour and activity as at the com- 

 mencement. Thus we see that teeth possessed of roots, obtain them at periods of 

 their existence more or less advanced. Among the herbivorous animals, the Horse 

 for example, the vitality of the bulb continues for several years, while it ceases in a 

 very short time with the Carnassiers. In this respect, the Mammalia offer a great 

 Tariety of examples. 



There are several circumstances which serve to confirm the accuracy of these 

 views. As long as the dentary capsule is wholly occupied in depositing the crown, 

 we see, at the precise point where the membranes composing it reunite and become 

 confounded, a uniform disc, supplied with an immense number of vessels, which 

 distinguish it readily from all the adjacent parts. It is from this surface that the 

 capsule continues to grow uniformly^ until the crown has acquired its entire height. 

 At the latter period, however, it undergoes a total change ; the isolated portions of 

 the vessels disappear, and those which remain compose little circles, more or less 

 numerous, and distinct from each other. From these circles the roots grow ; during 

 which operation, the external membrane detaches itself from the bulb on all the in- 

 termediate points of the partial cu-cles. The crown is then terminated by the de- 

 position of ivory between the roots and beneath both the crown and the bulb ; further, 

 M this deposition takes place from different points of the circumference of the tooth, 

 22 



it is at the intx-^rnal surface of the roots that it reunites. The little circles continue 

 to diminish ; sometimes they divide after a certain growth has taken place in the 

 root, causing tht- ra to appear more or less forked ; and they end in disappearing gra- 

 dually, so as to occasion all the roots to terminate in a point or thin layer. By 

 this growth, the bulb, now reduced within very narrow dimensions, remains inclosed 

 within the crown, and the roots are found to be pierced through their entire length 

 by those vessels and nerves which formed them ; thus connecting them with the bulb 

 on the one hand, and oa the other with the maxillary vessels and arteries. 



The first traces of the dentary capsule can be discovered in the fcetus, it is said, 

 during the earliest days of its life. There can be no doubt, however, that the teeth 

 are in a great measure formed at the period of birth in a large portion of the Mam- 

 malia, and the young animals are even compelled to use them before the period of 

 their lactation has entirely terminated. Physiologists are not, however, agreed as 

 to the nature of the process carried on within the jaws, in those parts which are 

 traver.'ied by the teeth, before leaving the gums. Some hd,ve supposed, that there 

 exists a natural passage, leading from the capsule, out of the gums ; and it is imagined, 

 that this cavity is enlarged by the expansive force of the tooth, aided by the elasticity 

 of the adjacent parts. Others have conceived, that the tooth tears everything 

 which opposes its passage; and they have even attributed to this cause, some of the 

 accidents which occasionally accompany the dentition of young animals. 



The former of these views is opposed by the observed phenomena of the secona 

 dentition, where another set of teeth is developed immediately beneath the first, in 

 such a way, that the second cannot appear before the first have fallen. No such na- 

 tural passage has been observed; and it ought not to be presumed before adequate 

 proof, that Nature has employed two different methods of evolving these organs, h 

 should rather be inferred, that if the second teeth are able to surmount the obstacles 

 presented to their growth by the first teeth immediately above them, these will also 

 be able to overcome the resistance of the membranes and cartilages, when they are 

 required to leave the jaws, to satisfy the new wants of the young animal. It further 

 appears, that teeth of the most complicated form, having their crowns terminated by 

 many tubercles, and having between them many intervals of considerable depth, ob- 

 trude therase'ives, by the summits of their tubercles, on several points at the same 

 time, beyond the gums; yet the gums still continue to occupy the intervals which se- 

 parate their tubercles. 



The hypothesis of a violent tearing is still less admissible than that of a natural 

 passage. During the time that the teeth are growing, not the slightest trace of such 

 a phenomenon can be observed; and analogy does not appear to justify this second 

 supposition. Nature appears to have a surer and more effectual means than these 

 mechanical hypotheses would lead us to infer; for the present is, in reality, only a 

 particular case of a very general law, of which it forms one of the most exact appli- 

 cations. 



There is no truth in Physiology better established than this, that the nutritive 

 power of any organic part is enfeebled, when it receives the continued mechanical 

 action of any foreign body whatever, and the nutrition of the part may even be wholly 

 interrupted, if this action acquire a certain degree of intensity. It seems, that in the 

 perpetual interchange of particles which constitutes life, the new molecules become 

 incapable of replacing the former, whenever a foreign body compresses the parts from 

 which the others have escaped. It may be said, either that the place of the first bodies 

 has ceased to be occupied, or that the assimilating force, which ought to have sup- 

 plied new molecules, has ceased to act. The consequence is, that the part becomes 

 obhterated; and the molecules, which should have nourished it, are dissipated, or go 

 to supply the adjacent parts. 



There can be little doubt that the development of the teeth is a phenomenon of 

 this description. When the crown of a tooth begins to be formed, and still more, 

 before this period, all that part of the gum, which is intended to be opened for its pas- 

 sage, is thick and filled with vessels. As the tooth grows, this part becomes smaller, 

 and the time at length arrives, when it consists of nothing more than a compact and 

 dry skin, which soon disappears in order to allow a free passage to the tooth. This 

 view of the subject is, however, incapable of explaining how it happens that the 

 pressure of the teeth is exerted contrary to the gums, rather than in the opposite 

 direction. Although the tooth begins at first to form only on the side next to the crown, 

 this circumstance does not completely account for the fact that the tooth tends exclu- 

 sively to emerge on this side. The reaction of a tooth growing in the direction of its 

 root, is equal to its action in the direction of the crown ; and if the degree of firm- 

 ness possessed by the adjacent parts be regarded, in this question, instead of piercing 

 the gums, the teeth ought to descend on the side where the roots are afterwards 

 found ; for the inferior parts of the capsule and its bulb would offer much less resist- 

 ance than the denser structure of the gums. It is therefore probable that we ought 

 to attribute the natural direction of the teeth to some special impulse which the cir- 

 culation impresses upon the dentary organ, as well as to the mere growth of the 

 capsule by its interior part. The addition of matter to the inner extremity of the 

 crown is far from being sufficient of itself to explain this phenomenon. The pressure 

 of the gum upon the teeth would even be sufficient wholly to arrest it, and it is, on 

 the contrary, the life of the gum which would then have to be suspended. While 

 the teeth are growing, the vital action of their capsules is raised to an intense degree, 

 the blood is directed towards them with great force, their irritation becomes extreme, 

 and hence probably result the fatal consequences which frequently occur to young 

 animals during the period of their dentition. 



The protrusion of the teeth from the sockets, in consequence of their secretion and 

 growth, is not the sole movement which these organs present. Other changes suc- 

 ceed, the object of which being the mastication of the food, is rather more obvious 

 than the causes which produce them. 



Among these may be considered, in the first place, the secondary movement of the 

 crown in teeth with distinct roots, after they have emerged from the sockets. The 

 capsules of all these teeth being entirely inclosed within the jaws, have their lower 

 parts, which correspond with the neck of the tooth, much below the dentary margin 

 of these bones ; but when the teeth are entirely formed, the neck is on a level with 

 this same border — that is to say, the inner part of the crown, which in some manner 



