752 EXAMINATION FOE :AGE. 



The milk incisors differ from the permanent ones by being 

 shorter and whiter ; having a better defined neck ; having their 

 exterior surface smooth, while that of the others generally has a 

 slight vertical groove; and from the fact that the milk incisors 

 gradually becx)me shorter and shorter, as soon as they come into 

 wear, the opposite being the case with the permanent ones. 



Form of the Teeth. 



The form of the milk nippers is shown in Fig. 200 (p. 754). The 

 permanent incisors (Fig. 201) taper gradually down to the end of 

 the root, when viewed from the front, or from behind; although 

 looking at them in profile, they are somewhat thicker away from 

 the crown than on it. Hence, the table, which at first is broad 

 from side to side, and narrow from front to rear, becomes in time, 

 as it gets worn down, narrower and narrower from side to side, 

 and slightly broader from front to rear. This is well shown by 

 Fig. 202 (p. 755). Also compare Fig. 245 (p. 793) with Fig. 269 

 (p. 809). 



Structure of the Teeth. 



The teeth — like scarf-skin, hair and horn — are a special form of 

 epithelium (pp. 154 and 190). Hairs and teeth are organs in all 

 respects homologous, and true dermal organs. Substitute corneous 

 matter for calcareous, and the tooth would be a hair. 



The body of a tooth (Fig. 203, p. 756), is composed of an ivory- 

 like substance called dentine, and has a hollow (the pulp-cavity) ex- 

 tending from its base up its centre, in which cavity the blood-vessels, 

 nerves, and secreting cells, concerned in the nourishment of the 

 tooth, are lodged. The dentine is more or less covered by a layer 

 of white and very hard material termed enamel, with which the 

 animal cuts and masticates his forage. In th© milk incisors (Fig. 

 200), the enamel does not extend below the crown. In all the 

 permanent teeth, the enamel also covers the greater part of the 

 root. Over the whole surface of each unused tooth, there is an 

 envelope of cement, which is neajrly similar in structure to bone, 

 and which can be regarded only as a covering, not as a portion, 

 of the tooth, because its development is different from that of the 

 dentine and enamel.* On the upper surface of the incisors, the 



• Cementum is developed, just as bone is, in two distinct methods. 



Where it is not to be very thick, and .is to clothe roots, the ossification 

 takes place in membrane (the alveolo-dentar periosteum), but where it is 

 to form a thick layer over the crown, as in Ruminants, a cartilaginous 

 cement organ is formed, and we have a calcification analogous to formation 

 of bone in cartilage. Thus the cement organ is found in those animals 

 only which have coronal cement, such as the Herbivora. 



