THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 125 



carnivores, the latter with the chisel-like front teeth and grinding back teeth 

 common to rodents. 



Cat: At the tip of the jaws are six small simple teeth, named incisors. On 

 either side of the incisors is a canine, a long, sharp but simple tooth. Back of 

 the canine on each side are four teeth in the upper jaw, three in the lower. These 

 teeth are mostly more complicated than the preceding, having more than one 

 cusp or pointed projection and more than one root. These teeth are known as 

 premolars and molars. In the upper jaw the first three on each side are pre- 

 molars, and the first two in the lower jaw; the last tooth on each side in each 

 jaw is a molar. Note that the upper incisors are borne on the premaxillae; the 

 other teeth of the upper jaw on the maxilla. Between the canines and the 

 premolars is more or less of a gap, known as a diastema. 



Rabbit: At the tip of the premaxillae are borne four chisel-like incisor teeth, 

 a small pair behind a larger anterior pair. The chisel-like form of the incis- 

 ors is characteristic of rodents and is due to the fact that the enamel is 

 present on the front face of the tooth only; the posterior face being composed 

 of the softer dentine wears away, leaving a sharp edge to the enamel. The 

 incisors of rodents further continue to grow indefinitely so that the loss from use 

 at the tip is replaced by growth at the root. At the tip of the lower jaw are two 

 similar incisors. Posterior to the incisors is a very wide gap or diastema. The 

 canine teeth, found in most mammals posterior to the incisors, are missing in 

 the rabbit and rodents in general. On the alveolar process of the maxilla are 

 borne six teeth in the upper jaw and five in the lower, on each side. Of these 

 the first three in the upper and first two in the lower jaw, are premolars, the last 

 three molars. The teeth are ridged crosswise, an adaptation for the grinding 

 of vegetable food; the ridges consist of enamel with dentine between them. 



The teeth of mammals are generally designated by a dental formula, which 

 expresses the number of each different kind of tooth in each half-jaw from the 

 anterior to the posterior extremity of the jaw. The teeth of the upper jaw are 

 placed in the numerator of the formula, those of the lower jaw in the denomina- 

 tor. The complete dentition for mammals consists of three incisors, one canine, 

 four premolars, and three molars in each half of each jaw; the dental formula 

 is then 3/3, 1/1, 4/4, 3/3. In the cat the formula is 3/3, 1/1, 3/2, 1/1; in the 

 rabbit 2/1, 0/0, 3/2, 3/3; in man 2/2, 1/1, 2/2, 3/3. There is no structural 

 difference between premolars and molars, but those teeth with complicated 

 crowns which occur in both the "milk" and the permanent dentitions are 

 designated premolars, while those which are permanent from the first and never 

 replaced are designated as molars. It is not known whether the complicated, 

 apparently compound structure of premolars and molars is due to their having 

 arisen through the fusion of several simple teeth or to the subdivision of the 

 original simple conical teeth. (See further, K, pp. 228-31.) The structure of 

 teeth and their origin from placoid scales was discussed in Section IV. 



