THE MAMMALIA— MAN AND BEASTS. 



iU appearance ; finally, the first dentition is terminated by a second Molar, It must 

 be remarked that Molars, and not false Molars, immediately follow the Canine ; and 

 this is contrary to what is observed in the final dentition of the human species. The 

 general law, of which this is only a particular case, will come afterwards to be ex- 

 plained. 



The formula for the Milk-teeth, or first teeth of the Infant, is therefore 



2 + C + 2 M 



= T^ =20 



I 2 + C -h 2 M 



All '.he teeta of tne first dentition fall exactly in the same order as they have ap- 

 peared. The Incisors and Canines are replaced by Teeth of the same nature as 

 themselves, only stronger and larger than the first. On the contrary, the two first 

 Molars are replaced by false Molars only. This operation is finished towards the 

 twelfth year, and the first tuberculous molar, being the third in the order of time, 

 shows itself about the sixteenth or eighteenth year. This tooth is larger than any 

 which preceded it, or even than those which follow. Finally, the last of these teeth, 

 commonly termed wisdom teeth, and which may not improperly be styled a third 

 dentition, make their appearance a few years later, though sometimes they are de- 

 iaved as long as the thirtieth year, 



AH the teeth of the second dentition are formed, according to the important re- 

 searches of M. Serres, by the vessels and nerves of a special dentary canal, developed 

 beneath the first, and which replaces it as soon as the first set of teeth begins to fall. 

 We may thence conjecture that, at the time when the other Mammalia change their 

 teeth, an analogous phenomenon takes place. 



When the teeth of the first dentition fall, it is observed that the greater part ot 

 them have lost their roots, and that the lower part of the crown is tinged black, 

 and covered with asperities which seera to be the effect of a species of corrosion. It 

 will be proper, however, to postpone further notice of this curious phenomenon until 

 the subject of successive dentition has been concluded. 



The Apes of both continents present nearly the same phenomenon as the human 

 species, in respect to their first and second dentition. The Makis and Insectivora 

 have not yet been studied in these respects, but some Carnivora have been satisfac- 

 torily examined, and especially the Dogs and Cats, in which two- dentitions are 

 recognised. 



The first dentition of the Cats consists in the upper jaw of three incisors, one 

 Canine, one rudimentary false Molar, one carnassier, and one small tuberculous 

 Molar ; in the lower jaw of three incisors, one Canine, one false Molar, and one 

 carnassier. Or, 



^[ 3-1- C + (F-fC^+ 1) M .^ _14 

 13 + C + (F+ C) M 12 



In the second dentition of the Cals, the incisors and canines are replaced without 

 any important changes, by teeth similar to them in nature. It is also the same with 

 the first two false fliolars ; but the carnassiers are replaced by the second false 

 Molars, and both are developed immediately after the others, so that, from being 

 second Molars in the first dentition, the carnassiers pass onwards to be third Molars 

 in the second ; that is to say, in the upper jaw, the carnassier Molar has taken the 

 place of the tuberculous Molar, which in the second dentition appears in the fourth 

 and last place, while the carnassier of the lower jaw is developed in the place where 

 no tooth was found in the first dentition. This will be readily understood on com- 

 paring the following formula of the second and adult dentition of the Cats with the 

 preceding. 



js 4- C + (2F + CO M 14 



In the dentition of the Dogs we find phenomena very analogous to the above. On 

 completing their first dentition, they have in the upper jaw three incisors, one Canine, 

 one false Molar, one carnassit^r, and one large tuberculous jMolar ; in the lower jaw, 

 three incisors, one Canine, two false Molars, and one carnassier. The formula for the 

 first dentition of the Dogs is therefore 



= 26 



-^ = 30 



2j 3 + C -f (F + C'+ 1) M 



a + C + C'-^ F + CO M 



= i±:^2S 



14 



In the same manner as among the Cats, we find that the incisors and canines of the 

 Dogs are renewed, at the second dentition, in both jaws, without any important change. 

 Immediately after the canine, in the upper jaw, there appears a rudimentary false molar 

 in a spot where no tooth had previously existed. The false molar of the first dentition 

 is replaced by a tooth similar to itself; the carnassier by a third false molar, and the 

 tuberculous molar by a carnassier. Finally, a tuberculous molar, with a second or 

 smaller one, appears after the carnassier. In the lower jaw, as in the upper, there 

 occurs a rudimentary false molar after the canine. The two false molars of the first 

 dentition are replaced by teeth which resemble them, and the carnassier by another 

 false molar. Next follows the new carnassier, and immediately behind it, one lar^-e 

 tuberculous molar, and one rudimentary, in a place where no tooth had appeared 

 during the first dentition. These changes appear obvious to the sight, on contrasting 

 the following formula of the adult dentition in the Dogs with that last given. 



2.3 + C + (3F-I- C'-f 2) M 



3 + C + (4 F -f- C -I- 2) M 



— ^ — 40 



It follows from this, that in the second dentition of the Dogs and Cats, the teeth 

 are not only more numerous, but the carnassier molars are placed at a muCh greater 

 distance from the canine teeth than in the first dentition. 



This observation is equally applicable to all Carnassiers, and it is not difficult to 

 perceive the design of Nature, in thus altering the position of these teeth, which are 

 most important to animals feeding almost exclusively on flesh. To render the action 

 of these teeth always powerful, they are brought nearer to the fulcrum or hinge of 

 the jaw ; and thus the effect which the jaw would otherwise produce from its growth, 

 in diminishing their power, is cfToctuaHy counteracted. 



As the Rodentia do not possess different kinds of molars, they cannot present 

 these changes which we observe in the Carnassiers. Excepting the Cabiais {Hydro- 

 ehcETUs), their teeth of the second dentition are developed immediately under those 

 of the first, and the latter entirely resemble the former. The Cabiais, on the con- 

 trary, possess a peculiar mode of dentition, in common with the Elephants and Ethio- 

 pian Hogs (Phacochcerus). 



It has not yet been ascertained, whether the incisors of the Rodentia fall, and are 

 replaced. The Baron Cuvier has proved, that all species of Rodentia which have 

 only three molars, possess only a single dentition ; and that there is a second denti- 

 tion only among those species which have more than these three teeth ; that is to say, 

 all molars surpassing three in number, and placed before them in the jaws. He has 

 further made the singular observation, that the teeth of the first dentition fall, in 

 the Guinea Pigs, while these animals are yet in the womb of their mother. In spe- 

 cies of the Hare genus (^Lepns), the first teeth fall a few days after birth, and this 

 phenomenon is found even in those rudimentary incisors, which are known among all 

 animals of this genus to develop themselves behind the principal incisors. 



We proceed to the Pachydermata, as the Edentata have not yet offered any im- 

 portant observation in this branch of the subject. The first dentition of the Hippo* 

 potamus consists of two incisors and one canine in each jaw, of three false molars 

 and three tuberculous molars in the upper jaw, and of two false molars and three 

 tuberculous molars in the lower. Or, 



2|2 + + (3 F + 3) M I 



|2 + C + (2 F + 3) M 



= -nr = 3-^ 



In the second dentition, the incisors and canines of both jaws experience no change. 

 The first of the three false molars in the upper jaw falls, and is not replaced; the 

 two others are replaced by teeth the same as themselves, and a false molar succeeds 

 to the first real molar. But, at the same time, another molar is developed at the 

 extremity, so that the number of these real molars remains always the same, not- 

 withstanding the fall of the first real molar. In the lower jaw, the first false molar 

 falls without re-appearing ; the two following are replaced by teeth of the same kind, 

 and then, as in the other jaw, the last molar appears. The second dentary system 

 of the Hippopotamus may, therefore, be represented thus ; 



21 2 -I- C -K4 F + 3) M 



|2 + C + (2 F -f 4) M '' 



20 

 18 





We may apply the same observation which has .ilready been made regarding the 

 Carnassiers to the Hippopotamus ; and it is, probably, for a similar reason, that the 

 first real molar of the first dentition is replaced by a false molar in the second. 



The Ethiopian Hogs (^Phacoch<£rus) exhibit a new mode of change, which they 

 possess in common with the Cabiais (^Hydroch(vriis). Their last molar possesses a 

 movement from behind towards the front, so that, when entirely grown, the two 

 small teeth which preceded it have disappeared, and it alone occupies the jaws. 



The Elephants have a mode of dentition resembling the Cabiais and Phacochjerus. 

 Their molars begin to show themselves by the fore part, and continue to advance 

 from behind forwards ; fiom which it follows, that these animals have at first only 

 one molar in each jaw, afterwards two, then only one, then two again, and so on. It 

 appears that this movement is the consequence of the growth of eight teeth. The 

 first, which occurs soon after birth, has not fallen when the second makes its appear- 

 ance. At the age of two years, the latter remains alone; aqd this continues until 

 the appearance of the thir ', which remains alone until the sixth yeai-. At nine years 

 of a<Te, it also disappears to give place to the fourth, and so on. It should be noticed, 

 that these teeth always appear at first by their fore part, which, on that account, is 

 much sooner worn out than the hinder. 



Passing to the Horses, we return to the mode of dentition first described. The 

 teeth of the second dentition develop themselves immediately under such of the first 

 as are intended to fall; that is to say, under all the incisors, the canines, and the 

 first three molars. The only particulars requiring notice at present are, that the first 

 teeth are narrower than tho^e which succeed, and that the last molars appear as soon 

 as the first have fallen. ^ 



The Ruminantia present analogous phenomena. All the incisors ajid canines of 

 the first dentition give place to teeth of the same nature as themselves; and of the 

 six molars found in each jaw, the first three are replaced by the same kind of teeth, 

 beinw only less complicated. It is then, also, that the last molars, which are very 

 complicated, present themselves ; and we may thus perceive the same design of Na- 

 ture, already noticed, in respect to the Carnassiers and others. 



Among all these animals, the greater part of the teeth, at the time of falUng, pre- 

 sent nearly the same appearance as those of Man. Their roots have disappeared ; 

 and from the irregularities in these teeth at their innermost surface, it might be im- 

 agined that they had been corroded : and further, that being composed of different 

 substances, some of them were less accessible than others to the action of the corrosive 

 substance. As spots cr black stains are perceived on the whole extent of this surface, 

 they might appear to exhibit manifest traces of a kind of corrosion, and forcibly recall 

 the appearance of caries, or decay of the teeth, as has often been remarked. 



Many hypotheses have been proposed to explain this singular phenomenon. Yet 

 it is not easy to perceive how a corrosive fluid, if it really existed, could spare the 

 adjoining parts, and especially the teeth of the second dentition. The mechanical 

 action of the second teeth upon the first has also been supposed, and of all attempts 

 to explain the phenomena, this is undoubtedly the most unfortunate. One tooth can- 

 not v.-ear out another without wearing itself out likewise ; and the second teeth are 

 always in their most unblemished state at the time when the first have fallen: finally, 

 this has been attributed to a power of absorption ; and the last opinion is most gene- 

 rally adopted. It is not unlikely that caries, or decay of the teeth, may be produced 

 bv a similar cause. 



From the observations which have already been made, on the complication of the 

 dentary capsules, on the variety of substances of which the teeth are composed, on 

 the care which Nature takes in supplying the places of those teeth destined to fall, 

 on the different situations which they occupy, and the names they have received, will 



