130 OUTLINES or EQUINE ANATOMY. 



nent. These are distinguisliable by the following charac- 

 teristics : — Milk teeth are snialler and whiter than perma- 

 nent, and their infundibuluin is more shallow ; they have a 

 marked neck, and are smooth on the anterior surface of the 

 crown ; permanent incisors decrease in size gradually from 

 the nipping surface to the extremity of the fang, and pre- 

 sent grooves on their anterior surface, running in a direc- 

 tion from above downwards. At an early period of foetal 

 life there may be distinguished the smooth margin of the 

 jaw covered by mucous membrane. A longitudinal groove 

 shortly presents itself, and at the bottom of this "primitive 

 dental groove " mucous papillse appear. These are pro- 

 duced by a thickening of the mucous corium, which causes 

 an elevation of the epithelial and basement layers. Trans- 

 verse partitions (which subsequently become converted 

 into bone) are thrown across the groove, whereby il is 

 separated into as many cavities as it subsequently produces 

 teeth {follicular stage) . Then by gradual closure of the upper 

 part of the groove, by approximation of the margins, each 

 papilla becomes enclosed in a sac {saccular stage), which is 

 lined by epithelial cells. The parietal portion of the 

 epithelial layer is said to produce the crusta petrosa, 

 the visceral* (that clothing the papilla) epithelium is 

 converted into enamel, and the corium is continuously 

 developed into dentine, that part which has not yet been 

 changed being termed the dental pulp. The papilla thus 

 altered becomes too large for its cavity and forces its way 

 through the gum which forms the upper wall of the sac 

 {eruptive stage or " cutting the teeth''). Small portions of 

 the dental sac are separated from the main cavity at an 

 early stage of the developmental process, in those cases 

 where' a second or permanent set of teeth will be required 

 to replace the temporary organs. These small sacs are 

 termed cavities of reserve, and slowly undergo the changes 

 above described in connection with the parent sac, a papilla 

 being produced which subsequently becomes developed 

 into a permanent tooth, by the growth of which absorption 

 of the fangs of the temporary teeth is brought about, and 

 they, being deprived of their source of nutriment, fall from 

 the mouth {''shedding the milh teeth''). Of the molars, 

 the three anterior of each row only are replaced, for in the 

 jaw of the young animal there is not room enough for six 

 molars. Those teeth which occupy the three posterior 



