86 



FIRST CLASS OF THE VERTEBRATED ANIMALS. 



has been formed in the bottom of the jaws, ultimately finds itself on a level with their 

 exterior margin. This protrusion of the crown appears to be owing, at first, to the 

 growth of that part of the capsule which is about to give birth to the roots — a growth 

 which does not make its appearance until after the formation of the crown. Subse- 

 quently to this, it must be attributed to some special impulse of the circulation, 

 which maintains itself in a high degree as long as the dentary capsule preserves its 

 secreting power. Further, at this particular period of its growth, the gum no longer 

 opposes any resistance to the growth of the teeth. 



There is seen in the molars of the Horse a second kind of movement, which they 

 probably possess in common with all other herbivorous animals as well as the 

 Ruminantia. It consists in the continued obtrusion of their teeth, even when 

 completely formed, and opposed by others in the opposite jaw, against which 

 they act during mastication. This movement was fully demonstrated by Tenon 

 (Academie des Sciences, an. 6), who, however, did not investigate its cause. It 

 may be regarded as a continuous ossification of the jaws, — an operation which only 

 ceases with the hfe of the animal. In fact, a third movement of these teeth exhibits 

 this ossification tending continually to expel the teeth from their sockets ; and this 

 occurs when a tooth, not being opposed by others, is pushed out of the jaws. As no 

 force then opposes the continued secretion of bone, the sockets become filled, and 

 the teeth are driven from the place which they occupied, as if they were foreign 

 bodies. This movement, which is prejudicial to most animals, has one advantage 

 for those which are obliged to wear out their teeth in grinding their food ; for, al- 

 thou"-h the weaiing out of the teeth in these Mammalia is often very unequal, the 

 dentarv organs do not on that account remain uneven at their summits ; and the con- 

 sequence is, that the grinding of the food may be continued to the most advanced 

 periods of life. 



There is a secondary movement in the incisors or front teeth of the Rodentia 

 wholly opposite to the precedmg, and still more difficult to explain. That, part of 

 the tooth which corresponds in situation with the root, is placed much less towards 

 the front of the bones containing them, among young animals, than among the old. 

 These teeth continually fall back at their extremity where the bulb is placed, in pro- 

 portion as the animal grows, while they advance forward by the other extremity. 

 M. Frederic Cuvier, who observed this singular phenomenon in the Rabbit and 

 Guinea-Pi"-, supposes that the bulb continues to grow by its hinder part, being in- 

 fluenced by the nerves and vessels which thence derive their life ; and this pheno- 

 menon appears to be common to all teeth approaching to tusks in their general cha- 

 racter. 



Another problem connected with the incisors of the Rndentia is much less diffi- 

 cult to solve, we mean their curvature, and the peculiar curves which they affect. 



To produce an arched tooth, it is sufficient that the capsule be arched; but if the 

 curvature of the capsule remain always the same, these teeth, which can grow in- 

 definitely when no obstacle arrests their course, would present in this case the form 

 of a re"-ular circle, of which frequent examples are found. Instead, however, of this 

 curve, the incisors of the Rodentia exhibit one nearly approaching to a spiral, where 

 the first portions of the teeth are inclosed in those that follow. It is necessary, 

 therefore, that the capsule producing the teeth should change its curvature, and that 

 it should approach continually towards the right line, as these animals advance in 

 ftf^p up to a certain point, perhaps, when it ceases to be further modified. "We 

 may also remark, that these changes of curvature are the same in the incisors of 

 both jaws ; for these teeth, at all periods of life, preserve among themselves the 

 same relations. 



The appearance of the teeth beyond the gums usually commences, among the Mam- 

 malia with the period when the milk begins to be insufficient for the nourishment of 

 the youn'T animal ; but it very rarely happens that they are all developed at the same 

 time. In this respect, great differences are found among them ; and Nature, in most 

 cases, fads to impart at one time all the teeth necessary for the use of each animal. 

 There arc very few, we may almost say, no Mammalia, where some of these organs are 

 not renewed ; that is to say, that certain kinds of teeth fall, and are reproduced, or 

 rather replaced, once or oftener, by the successive growth of other teeth beneath, 

 behind, or before the former. 



These first teeth, which give place to new ones, are distinguished by the term 

 milk teeth, or teeth of the first dentition ; those which succeed are termed the second 

 t^etk. But these terms, founded upon what has been observed in the human species, 

 ouo-ht not to be taken in the strict sense when applied to other Mammalia ; for among 

 these it will be seen that the milk teeth may fall before birth, or a long time after 

 the adult af^e. To avoid the mistakes to which the ambiguity of these terms might 

 trive rise, it will be proper to employ the terras first, second, and third teeth, to de- 

 note the order of their appearance. 



This department of ISatural History, which shows the succession of the teeth, their 

 mutual influence, the coincidence of their appearance with other parts, and with the 

 new wants of each animal, as well as the relations of form and number between the 

 teeth of different dentitions, has, unforLunately, been much neglected, and it is only 

 now beginning to receive that attention which it merits. In a Zoological point of 

 view, a knowledge of the dentai-y system in different ages is almost indispensable ; but 

 we are still without a series of drawings, showing the teeth of young Mammalia, cor- 

 responding to the valuable lithographic sketches of M. Frederic Cuvier (Sur les Dents 

 des Mammiferes), made from adult specimens. 



Before explaining the few particulars that have hitherto been aseert:i.ined on this 

 subject, it will be proper to premise a few words respecting the different kinds of teeth, 

 as well as to explain the system of notation which we intend to \ise in describing 

 thera. 



The teeth of the Mammaha emerge solely from the inter-maxillary and maxillary 

 bones. The incisors, or front teeth, make their appearance first ; and these may be 

 followed either by the canines or molars. The last are subdivided into false molars, 

 carnassier molars, and tuberculous molars : while the tuberculous molars themselves may 

 be further distinguished by their having simple, compound, or proper tubercles. 



(1.) The Incisor, or Cutting Teeth (^Incisores or Primores), are somewhat broad 

 and long, with their margins often parallel, and cut away obliquely at their free ex- 



tremity. We see them so, for example, in the front teeth of Man ; but this form is 

 still more strongly marked in the Rodentia or Gnawers. This term Incisor ought 

 properly to be applied only to those teeth which have a form especially fitted for 

 cutting ; but it has been extended not only to all such as are found in the incisive 

 or inter-maxillary bone, but even to those opposed to them in the lower jaw, aUhou»h 

 the latter often have neithi-r the form nor use of true Incisors. 



(2.) The Canines (^Lajiiarii), or Tearing Teeth, have the general form of those 

 teeth which appear most prominently in the Dogs. They are longer than all the 

 others, and always have a single root and a single point to the crown. As these 

 teeth are usually placed in the upper and lower jaw, immediately behind the Incisors, 

 the term Canine has been extended to all teeth which appear to occupy this place. 

 They are likewise called Corner teeth ; and, from the chief use to which they aie 

 applied, have obtained the name of Laniarii, from laniare, to tear. 



(3.) As the names of most parts of animals are derived from the corresponding 

 parts in Man and the Ruminantia, which were principally dissected by the ancient 

 Anatomists, the term Molnr (^Molares) is correctly applied only to those which act, 

 as we may remark in the Ruminantia, almost like a mill-stone — (in Latin mola, a 

 mill). Hence, we understand Molar teeth to be compound, serai-compound, or even 

 simple teeth, having the crown broad and flat, with broken projections and small 

 eminences corresponding to each other in both jaws. Afterwards, however, the 

 term has come to be applied indiscriminately to all teeth situate behind the Canines, 

 and occupying the entire inner extremities of the dentary lines, although they some- 

 times possess no other character of molars than the place they occupy. Thus, in the 

 Cats, where these teeth are trenchant, and correspond in each jaw, so as, in fact, to 

 act in the same manner as true Incisors, they are not on that account deprived of the 

 common appellation of Molars. 



Hence it becomes necessary further to distinguish the different kinds of Molars 

 from each other. In many Mammalia, the Molars differ greatly both in size and 

 form, and have on that account been divided into False and True Molars, ^y false 

 Molars, being most coraraonly the anterior ones, we understand such as are small and 

 pointed. Trite Molars are considered to be thicker and larger, with their crowns 

 studded with several points, or altogether flat. In the Carnivorous animals, there is 

 found a very large ISIolar, which more especially fulfils the teai'ing purposes of these 

 animals ; and this tooth is further distinguished by the terms carnassier Molar. 



The importance of possessing a good system of Nomenclature for the teeth becomes 

 sufficiently obvious, as soon as the necessity of defining clearly the different kinds and 

 combinations of teeth, both in respect to their forms and relative position, has been 

 made apparent. 



In stating the dentary systems of animals, two methods of notation have hitherto 

 been employed ; for example, the adult teeth of the human species, being eight inci- 

 sors, four canines, eight false molars, and twelve tuberculous molars, have long been 

 represented thus : 



Incisor.s -■ ; Canines ■:-—, ; Molars -^^ = 32. 



In his work on the Teeth of the IMammaha, 

 same thing, under the following form : 



32 Teeth. 



16 upper. 



16 under. 



M. Frederic Cuvier expresses the 



f 4 Incisors. 



< 2 Canines. 

 t 10 Molars. 

 r 4 Incisors. 



< 2 Canines. 

 C 10 Molars. 



The former expression, besides its inconvenience from the smallness of the figures, 

 does not represent the nature of the Molars. The latter is not compact. In both, 

 the teeth on each side are confounded together unnecessarily. 



As a new system of notation, which will combine the advantages of brevity and 

 clearness, yet remains to be jiroposed, we venture to suggest the following, which 

 possesses, in our opinion, some of the most essential requisites. Let M represent 

 any Molar tooth ; C a Canine ; F a false Molar ; and C a carnassier j\Iolar. Let a 

 number annexed to an explanatory letter denote that there are as many teeth of the kind, 

 represented by the letter, as there are units in the number. Further, let a number 

 without an explanatory letter denote an Incisor, or front tooth, and a Molar, unless 

 otherwise expressed, be always understood to be tuberculous. Then, adopting the 

 ordinary signification of the Algebraical symbols, the dentary system of the Adult 

 Man will be conveniently lepresented as follows: 



2| 2 + C+ (2F + 3)M _ 16_ _ 

 I2 + C + (2 F + 3J M ~ 16 



where the numerator denotes the number, nature, and relative position of the teeth 

 on one side of the upper jaw, and the denominator of those on one side of the lower. 

 The small figure in the corner indicates that each expression must be doubled to 

 represent both sides of each jaw, and the vertical line on the left hand shows the 

 medial axis of the body, passing in the middle of the front teeth. 



To avoid repetitions, only one side of either jaw is alluded to in referring to par- 

 ticular teeth ; and what is said of one side must be understood of the other, which 

 precisely resembles the first in all its relations. It is always customary to count 

 from the anterior extremity of all the parts which bear these organs. Thus, the 

 first Incisor among the Mammalia is that tooth found nearest to the suture, by which 

 the inter-maxillary bones are united. This suture is represented in the formulae by 

 the vertical line. All extraordinary cases are excepted, such as the appearance of 

 teeth before birth or in extreme old age, while the ordinary and most natural process 

 of development is always understood to be meant, unless otherwise expressed. 



In the human species, the first dentition generally takes place from the sixteenth 

 or eighteenth month to the age of two years or two years and a half, and it usually 

 commences with the lower jaw. The first Incisors precede all the others, and these 

 are followed by the second; so that, towards the end of the first year, all the inci- 

 sors are developed. The first tooth which pierces the gums after the incisors is a 

 molar ; and it is subsequent to this that the Canine, though placed before it, makes 



32. 



