TEETH. 



897 



saurs. A thin compact, floor of bone separates 

 this groove, and the sockets anterior to it, 

 from the large cavity of the ramus of the jaw; 

 it is pierced by bloodvessels for the supply of 

 the pulps of the growing teeth and the vas- 

 cular dentiparous membrane which lines the 

 alveolar cavities. 



The tooth-germ is developed from the 

 membrane covering the angle between the 

 floor and the inner wall of the socket. It 

 becomes in this situation completely enveloped 

 by its capsule, and an enamel-organ is formed 

 at the inner surface of the capsule before the 

 young tooth penetrates the interior of the 

 cavity of its predecessor. 



The matrix of the young growing tooth 

 affects, by its pressure, the inner wall of the 

 socket, as shown in Jig. 573, and forms for 



Fig. 573. 



Section of lower jaw, with four alveoli and teeth, of 

 the black Alligator. 



itself a shallow recess : at the same time it 

 attacks the side of the base of the contained 

 tooth ; then, gaining a more extensive attach- 

 ment by its basis and increased size, it pene- 

 trates the large pulp-cavity of the previously 

 formed tooth either by a circular or semi- 

 circular perforation. The size of the calcified 

 part of the tooth-matrix which has produced 

 the corresponding absorption of the previously- 

 formed tooth on the one side, and of the 

 alveolar process on the other, is represented 

 in the second exposed alveolus of Jig. 573., the 

 tooth a having been displaced and turned 

 round to show the effects of the stimulus of 

 the pressure. The size of the perforation in 

 the tooth, and of the depression in the jaw, 

 proves them to have been, in great part, 

 caused by the soft matrix, which must have 

 produced its effect by exciting vital action of 

 the absorbents, and not by mere mechanical 

 force. The resistance of the wall of the 

 pulp-cavity having been thus overcome, the 

 growing tooth and its matrix recede from the 

 temporary alveolar depression, and sink into 

 the substance of the pulp contained in the 



VOL. IV. 



cavity of the fully-formed tooth. As the new 

 tooth grows, the pulp of the old one is re- 

 moved ; the old tooth itself is next attacked, 

 and the crown being undermined by the ab- 

 sorption of the inner surface of its base, may 

 be broken off by a slight external force, when 

 the point of the new tooth is exposed, as in 

 the/g. 573. b. 



The new tooth disembarrasses itself of the 

 cylindrical base of its predecessor, with which 

 it is sheathed, by maintaining the excitement 

 of the absorbent process so long as the cement 

 of the old fang retains any vital connection 

 with the periosteum of the socket ; but the 

 frail remains of the old cylinder, thus re- 

 duced, are sometimes lifted off the socket 

 upon the crown of the new tooth, as \njig. 573. 

 b, when they are speedily removed by the 

 action of the jaws. This is, however, the 

 only part of the process which is immediately 

 produced by mechanical force: an attentive 

 observation of the more important previous 

 stages of growth, teaches that the pressure 

 of the growing tooth operates upon the one 

 to be displaced only through the medium 

 of the vital absorbent action which it has 

 excited. 



Most of the stages in the development and 

 succession of the teeth of the crocodiles are 

 described by Cuvier with his wonted clearness 

 and accuracy ; but the mechanical explanation 

 of the expulsion of the old tooth, which 

 Cuvier adopts from M. Tenon, is opposed 

 by the disproportionate smallness of the hard 

 part of the new tooth to the vacuity in the 

 old one, and by the fact that the matter im- 

 pressing viz. the uncalcified part of the 

 walls of the tooth-matrix is less dense than 

 the part impressed. 



No sooner has the young tooth penetrated 

 the interior of the old one, than another 

 germ begins to be developed from the angle 

 between the base of the young tooth and the 

 inner alveolar process, or in the same relative 

 position as that in which its immediate pre- 

 decessor began to rise, and the processes of 

 succession and displacement are carried on, 

 uninterruptedly, throughout the long life of 

 these cold-blooded carnivorous reptiles. 



From the period of exclusion from the egg, 

 the teeth of the crocodile succeed each other 

 in the vertical direction ; none are added 

 from behind forwards, like the true molars in 

 Mammalia. It follows, therefore, that the 

 number of the teeth of the crocodile is as 

 great when it first sees the light as when it 

 has acquired its full size; and, owing to the 

 rapidity of the succession, the cavity at the 

 base of the fully-formed tooth is never con- 

 solidated. 



The fossil jaws of the extinct Crocodilians 

 demonstrate that the same law regulated the 

 succession of the teeth, at the ancient epochs 

 when those highly organised reptiles prevailed 

 in greatest numbers, and under the most 

 varied generic and specific modifications, as 

 at the present period, when they are reduced 

 to a single family, composed of so few and 

 slightly varied species as to have constituted 

 3*i 



