STEUCTURE OF THE TEETH. 74J 



The milk incisors differ from the permanent ones by being 

 shorter and whiter ; having a better defined neck ; ha^dng their 

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

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

 gradually become 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 sho'^m in Fig. 195. The 

 peniianent incisors (Fig. 196) 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. Tliis is well shown 

 by Fig. 197. Also compare Fig. 240 with Fig. 264. 



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 time dermal organs 



Substitute corneous matter for calcareous, and the tooth would 

 be a hair " {Huxley). 



The body of a tooth (Fig. 198) is composed of an ivory-like 

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

 extending from its base up its centre, in which ca"\dty the blood- 

 vessels, nerves, and secreting cells, which are 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 the 

 milk incisors (Fig. 195), 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 nearly similar in 

 stinicture 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 



* " Cementum is, according to Messrs. Legros, Robin, and Magitot, 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 



