DATES OF TEETHING. 



757 



for the teeth, which, then, more or less radiate outwards. As the 

 teeth wear down, they obtain more and more room, on account 

 of their becoming narrower from side to side ; and the jaw, conse- 

 quently, '■ falls in." 



6. Owing to the irritation set up, by the movement of the incisor 

 teeth in their sockets, when they become short with age, the lining 

 membrane (periosteum) of the sockets sometimes secretes an excess 



B c 



Fig. 204. — Transverse section of a lower right central incisor, showing 



the difi'erent layers of the tooth, with their relative thickness 



(enlarged). A, front face. B, rear face. C, external cement, 



c', central cement. E, external enamel. E', central enamel. 



I, external dentine. I', internal and darker coloured dentine. 

 ( FroDi Goiibaux and Barrier. J 



of cement around the stumps, which helps to keep these stumps 

 in their place, and to increase the area of their cutting surface. 



Dates of Teething. 



As a rule, the incisors of the upper jaw make their appearance 

 sooner than those of the lower Jaw. At birth, the central milk 

 incisors show themselves under the gum (Fig. 220, p. 774), and 

 come through it about a week later on. The lateral milk incisors 

 show through the gum at the age of about five weeks ; and the 

 corner milk incisors, when the animal is about nine months old. 

 The eruption of the central, lateral, and corner permanent incisors 

 respectively takes place at from 2| to 3, 3J to 4, and i^ to 5 years 

 after birth (Cornevin and Lesbre). 



