288 COMPARATIVE ANATOMY 



If we now examine the teeth of the cat, we find that they 

 are not unhke those of the dog in shape, but are fewer in 

 number. There are only three premolars and one molar in 

 the upper jaw, and two premolars and one molar in the lower 

 jaw on each side. The molar tooth of the upper jaw is small 

 and rudimentary, but the third premolar is very large, has a 

 sharp trenchant edge, and works against the similarly modified 

 molar of the lower jaw. The arrangement of the teeth of a 

 mammal may be represented by a dental formula, that of the 

 dog being 



If Ci PM| Mf, 



in which the letters I., C, P.M., M., stand for incisors, 

 canines, premolars, and molars. The upper row of figures 

 shows the number of each kind of tooth in the upper jaw, 

 the lower row the number in the lower jaw. As the teeth of 

 the two sides of the mouth are similar, those of one side only 

 are represented in the formula. 



The dental formula of the cat is therefore 



If CI PM| Mi 



The teeth of the rabbit differ a good deal in appearance 

 from those of the cat and dog. They are implanted in 

 sockets ; but their bases do not taper off to form fangs, and 

 their pulp-cavities are widely open below. The pulp remains 

 active throughout life, and the teeth, as they wear away above, 

 are constantly replaced from below. 



The dental formula is 



If C^ PM# M4. 



The second incisors of the upper jaw are rudimentary, and 

 are lodged in sockets immediately behind the large curved 

 anterior incisors. The canines are absent, and the incisors 

 are separated from the premolars by a wide space known as 

 a diastema. The premolars and molars have flat grinding 

 crowns, marked by transverse ridges, formed by the infolding 

 of the enamel of their outer surfaces. 



In the alimentary canal of mammals the stomach very rarely 

 retains the primitive tubular shape seen in the dogfish and 

 frog, but is generally transversely elongated, and it may be 

 complicated by sub-division into several chambers, more 

 especially in ruminating animals such as oxen. The large 



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