TEETH. 



899 



stricted growth, which fang gradually tapers 

 to its extremity ; those teeth which grow un- 

 interruptedly have not their exposed part se- 

 parated by a neck from their implanted part, 

 and this generally maintains to its extremity 

 the same shape and size as the exposed 

 crown, 



It is peculiar to the class Mammalia to have 

 teeth implanted in sockets by two or more 

 fangs ; but this can only happen to teeth of 

 limited growth, and generally characterises 

 the molars and premolars ; perpetually grow- 

 ing teeth require the base to be kept simple 

 and widely excavated for the persistent pulp. 

 In no mammiferous animal does anchylosis of 

 the tooth with the jaw constitute a normal 

 mode of attachment. Each tooth has its 

 particular socket, to which it firmly adheres 

 by the close co-adaptation of their opposed 

 surfaces, and by the firm adhesion of the 

 alveo-lar periosteum to the organised cement 

 which invests the fang or fangs of the tooth ; 

 but in some of the Cetacea, at the posterior 

 part of the dental series, the sockets are wide 

 and shallow, and the teeth adhere more 

 strongly to the gum than to the periosteum ; 

 in the Cachalot I have seen all the teeth 

 brought away with the ligamentous gum, when 

 it has been stript from the sockets of the 

 lower jaw. 



Teeth are fixed as a general rule in all 

 Vertebrata, and the only known exceptions 

 are those presented by certain species of 

 fishes, e. g. the Sharks, Lophioids, Gonio- 

 donts. In the higher Vertebrata the move- 

 ments of the teeth depend on those of the 

 jaw-bones to which they are affixed, but appeal- 

 to be independent in the ratio of the size of 

 the tooth to the bone to which it is attached. 

 Thus the extent of rotatory movement to 

 which the large perforated poison fangs of 

 the rattle-snake are subject depends upon the 

 rotation of the small maxillary bone. So 

 likewise the seemingly individual movements 

 of divarication and approximation observable 

 in the large lower incisors of the Bathyergus 

 and Macropus *, are due entirely to the 

 yielding nature of the syinphysis uniting the 

 two rami of the lower jaw in which those 

 incisors are deeply and firmly implanted. It 

 is no more a property of the teeth themselves 

 than is that alternate removal of the lower 

 teeth from, and bringing of them in contact 

 with, the upper teeth of the mouth, which 

 one sees or feels in the act of mastication. 



True teeth implanted in sockets are con- 

 fined, in the Mammalian class, to the maxil- 

 lary, premaxillary, and mandibular, or lower 

 maxillary bones, and form a single row in 

 each. They may project only from the pre- 

 maxillary bones, as in the Narwhal, or only 

 from the lower maxillary bone, as in Ziphius ; 

 or be apparent only in the lower maxillary 

 bone, as in the Cachalot ; or be limited to the 

 superior and inferior maxillaries. and not pre- 

 sent in the premaxillaries, as in the true 

 Pecora, and most Bruta of Linna3iis ; in 



* See Mason Good's Book of Nature, vol. i. p. 285. 

 1826. 



general, teeth are situated in all the bones 

 above mentioned. In Man, where the pre- 

 maxillaries early coalesce with the maxillary 

 bones, where the jaws are very short and the 

 crowns of the teeth are of equal length, there 

 is no interspace or " diastema " in the dental 

 series of either jaw, and the teeth derive 

 some additional fixity by their close apposi- 

 tion and mutual pressure. No inferior Mam- 

 mal now presents this character ; but its im- 

 portance, as associated with the peculiar 

 attributes of the human organisation, has been 

 somewhat diminished by the discovery of a 

 like contiguous arrangement of the teeth in 

 the jaws of a few extinct quadrupeds : e. g. 

 Anoplothcrium, Nesodon, and Dichodon.* 



The teeth of the Mammalia usually consist 

 of hard unvascular dentine, defended at the 

 crown by an investment of enamel, and every- 

 where surrounded by a coat of cement. The 

 coronal cement is of extreme tenuity in Man, 

 Quadrumana, and terrestrial Carnivora ; it is 

 thicker in the Herbivora, especially in the 

 complex grinders of the Elephant ; and is 

 thickest in the teeth of the Sloths, Mega- 

 therioids, Dugong, Walrus, and Cachalot. 

 Vertical folds of enamel and cement penetrate 

 the crown of the tooth in the Ruminants, and 

 in most Rodents and Pachyderms, charac- 

 terising by their various forms the genera of 

 the last two orders ; but these folds never 

 converge from equidistant points of the cir- 

 cumference of the crown towards its centre. 

 The teeth of the quadrupeds of the order 

 Bruta (Edentata, Cuv.) have no true enamel ; 

 this is absent likewise in the molars of the 

 Dugong and the Cachalot.-]- The tusks of 

 the Narwhal, Walrus, Dinotherium, Masto- 

 don, and Elephant consist of modified den- 

 tine, which, in the last two great proboscidian 

 animals, is properly called " ivory," J and is 

 covered by cement. 



In the subjoined magnified view of a section 

 of the molar of a Megatherium, t is the hard 

 dentine, v the vaso-dentine, and c the cement 

 (fig. 574). 



The teeth in the Mammalia, as in the fore- 

 going classes, are formed by superaddition of 

 the hardening salts to pre-existing moulds of 

 animal pulp or membrane, organised so as to 

 insure the arrangement of the earthy particles 

 according to that pattern which characterises 

 each constituent texture of the tooth. 



The complexity of the primordial basis, or 

 matrix, corresponds, therefore, with that of 

 the fully-formed tooth, and is least remark- 

 able in those conical teeth which consist only 



* Quarterly Journal of the Geological Society, 

 Feb. 1848, p. 36, pi. iv. 



t M. Fr. Cuvier divides tlie teeth of Mammalia, 

 according to their composition, into four classes: 

 the first consists of ivory (dentine), enamel, anil 

 cement ; the second of ivory and enamel ; the third 

 of ivory and cement; the fourth of ivory only. 

 (Dents des Mammiferes, p. xxi.) I have met with 

 no Mammalian teeth in which cement is absent, 

 and believe that the second and fourth of the above- 

 cited classes of teeth have no existence in nature. 



J " Hoc soluru est ebur." Plinius, Hist. Nat. lib. 

 xi. c. 37. 



3 M 2 



