292 



REPTILIA. 



The size of this perforation in the tooth 

 and of the depression in the jaw, compared 

 with that of the calcified part of the 

 tooth matrix, proves them to have been, 

 in great part, caused by the soft matrix, 

 which must have produced its effect by ex- 

 citing 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 cavity of the fully 

 formed tooth. As the new tooth grows, the 

 pulp of the old one is removed : the old 

 tooth itself is next attacked, and the crown, 

 being undermined by the absorption of the 

 inner surface of its base, may be broken oft' 

 by a slight external force, when the point of 

 the new tooth is exposed. 



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 con- 

 nexion with the periosteum of the socket ; 

 but the frail remains of the old cylinder, thus 

 reduced, are sometimes lifted out of the 

 socket upon the crown of the new tooth, 

 when they are speedily removed by the action 

 of the jaws. 



No sooner has the young tooth penetrated 

 the interior of the old one, than another germ 

 begins to be developed from the angle be- 

 tween 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 anJ 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 the animal first sees the light, as 

 when it has acquired its full size ; and, owing 

 to the rapidity of their succession, the cavity 

 at the base of the fully-formed tooth is never 

 consolidated. 



Tongue. In reptiles the tongue can scarce- 

 ly be regarded as an organ of taste, and in- 

 deed, seeing how little their teeth are adapted 

 for mastication, and how very generally their 

 prey is swallowed entire, such a sense could 

 hardly be usefully accorded to them. The 

 lingual apparatus is, therefore, variously modi- 

 fied in different genera, and converted into an 

 instrument of prehension in some cases, whilst 

 in others it seems to perform functions, the 

 nature of which is not so obvious. 



In the C/ielonian reptiles the tongue is but 

 little adapted to appreciate savours : it is 

 covered with a thick nigcse membrane, the 

 surface of which in the turtles is smooth, 

 but in some tortoises, as, for example, in the 



Testudo indica, it is remarkably beset with 

 numerous elongated and pointed papillae. 



Fig. 212. 



Tongue of the Cliameleon partially extended. 



a, slender extensible portion of the tongue ; b, c, 

 its bulbous extremity; d, e, f, prehensile apparatus 

 at the end of the tongue. 



Beneath the mucous membrane which covers 

 this organ, there exists, in this order, a thick 

 stratum of glandular follicles, whilst others 

 situated beneath the tongue pour out their 

 secretion through numerous ducts situated on 

 each side of the fraenuin linguae. The body 

 of the os hyoides is cartilaginous, and from 

 its extremity a glosso-hyal or lingual bone is 

 prolonged into the substance of the tongue. 



In those lizards which feed upon vegetable 

 substances, the hotly of the tongue is fleshy, 

 generally bifid at its extremity, and its surface 

 is papillose. In some instances, as in the 

 Iguana, its apex is tipped with horn. In 

 other genera of lizards, which feed principally 

 on insects, the extremity of the tongue is 

 fissured, and the whole organ is remarkably 

 extensible, forming an apparatus wherewith to 

 catch their insect prey. 



Perhaps in no animal is the tongue more 

 remarkable than in the Chameleon, where by its 

 extraordinary power of extension, and by the 

 rapidity of its movements, it is made to com- 

 pensate for the extreme sluggishness which 

 characterises the muscular system of that 

 animal. The chameleon, fixed firmly by 

 means of its bifid feet upon the bough of a 

 tree, and concealed as much as possible by 

 adopting the colours of the branches around, 



