COMPARATIVE ANATOMY. 



through the gum. They represent the 

 segment of a circle, and are contained in 

 a canal of the bone, which descends un- 

 der the sockets of the grinders, and then 

 mounts up, in some instances, to the root 

 of the coronoid process : hence, although 

 their anterior cutting edge is in the front of 

 the mouth, the posterior extremity is 

 behind all the grinding teeth. No ani- 

 mal exhibits this structure better than 

 the rat. The beaver also affords a good 

 specimen of it on a larger scale. It has 

 been drawn in this animal by Blake, 

 (" Essay on the structure, &c. of the 

 teeth.") The tooth does not extend so far 

 in the upper jaw ; it is there implanted in 

 the intermaxillary bone, and terminates 

 over the first grinder. 



The observations which have been 

 made respecting the constant growth of 

 the incisor teeth of the glires will apply 

 also to the tusks of the elephant. These 

 are hollow internally, through the greater 

 part of their length, and the cavity con- 

 tains a vascular pulp, which makes con- 

 stant additions of successive layers, as 

 the tusk is worn down. One of the ele- 

 phants at Exeter Change is said to. have 

 nearly bled to death from a fracture of 

 the tusk, and consequent laceration of the 

 vessels of the pulp. The tusks of the 

 hippopotamus, and probably all other 

 teeth of this description, grow in the same 

 marfner. Farther and more accurate ob- 

 servation may hereafter shew, that the 

 same mode of growth obtains also in 

 other classes of teeth, when they are ex- 

 posed to great friction. Something simi- 

 lar may certainly be observed in the grin- 

 ders of the horse. The tooth is not finish- 

 ed when it cuts the gum : the lower part 

 of its body is completed, while the upper 

 part is worn away in mastication ; and 

 the proper fang is not added till long 

 after. Hence we can never get one of 

 these teeth in a perfect state, for if the 

 part out of the gum is complete, the 

 rest of the body is imperfect ; and 

 there are no fangs : on the contrary, 

 when the fangs are formed, much of the 

 body has been worn away in mastica- 

 tion. Blake also asserts, that this struc- 

 ture is found in the grinders of the beaver, 

 (p. 99.) 



The narwhal is particularly distinguish- 

 ed by its long and spiral tusk. The ani- 

 mal is found so constantly with only one 

 tusk, that it has been called, in common 

 language, the sea-unicon ; and Linnxus 

 has even given it a similar appellation, 

 that of monodon. Yet there can be no 



VOL. III. 



doubt that it possesses originally two of 

 these; one in either jaw bone-, and that 

 which is wanting must have been lost by 

 some accidental circumstance, as we 

 can easily suppose, (" Shaw's Zoology," 

 vol. ii. p. 473.) These tusks often equal 

 in length that of the animal's body : 

 which may be 18 feet or more : yet they 

 are .always slender. 



In many baboons, and most particular- 

 ly in the larger predaceous mammalia, 

 the canine teeth are of a terrific size; in 

 the latter animals, the whole profile of 

 the anterior part of the cranium forms a 

 continuous line with these teeth ; which 

 is very visible in the tiger. The canine 

 tusks of the babiroussa, which are very 

 long, and recurved so as nearly to de- 

 scribe a complete circle, present the 

 most curious structure. Their utility to 

 the animal appears quite obscure, when 

 their length, direction, and smallness, are 

 considered. 



The distribution of the enamel and 

 bony substance varies in the teeth of 

 different animals, and even in the 

 different orders of teeth in the same 

 animal. 



All the teeth of the carnivora, and the 

 incisors of the ruminating animals, have 

 the crown only covered with enamel, as 

 in the human subject. The immense 

 fossil grinders of the animal incognitum, 

 or mammoth, have a similar distribution of 

 this substance. 



The grinders of graminivorous quadru- 

 peds, and the incisors also of the horse, 

 have processes of enamel descending 

 into the substance of the tooth. These 

 organs have also in the last-mentioned 

 animals a third component part, differing 

 in appearance from both the others, but 

 resembling the bone more than the ena- 

 mel. Blake has distinguished this by the 

 name of crusta petrosa ; and Cuvier calls 

 it cement. 



The physiological explanation of this 

 difference in structure is a very easy 

 and clear one. The food of the carnivora 

 requires very little comminution before 

 it enters the stomach : hence the form of 

 their grinding teeth is by no means cal- 

 culated for grinding : and as the articula- 

 tion of the jaw admits no lateral motion, 

 the molares, of which the lower are over- 

 lapped by the upper, can only act like the 

 incisors of other animals. The food of 

 graminivorous quadrupeds is subject to 

 a long process of mastication, before it is 

 exposed to the action of the stomach. 

 The teeth of the animals suffer gi-eat at- 



Pp 



