86 ANIMAL PHYSIOLOGY. 



filled with epithelium, and with a papilla at the bottom; 

 and in both the depression becomes temporarily converted 

 into a closed sac, which afterwards is burst open by tho 

 protrusion of the contained organ. The layer of epithelium 

 immediately in contact with the tooth-papilla is converted 

 into enamel, the enamel-columns corresponding with the 

 elongated cells of the deepset layer of the cuticle. The 

 dentine is formed from the superficial part of the papilla 

 itself, while the remainder of that structure constitutes the 

 pulp. The development of the tooth proceeds from the 

 summit of the crown downwards, the pulp thus becoming 

 gradually enclosed. The sacs of the permanent teeth make 

 their first appearance from the necks of the sacs of the milk 

 set at a very early period, while the latter are still open, and 

 subsequently descend to positions beneath the milk teeth, 

 which they are destined to replace. 



Fig. 47. DEVELOPMENT of a temporary and a permanent tooth 

 (Goodsir). a, papilla on the floor of the primitive dental groove; 

 0, papilla enclosed in a follicle in the bottom of the secondary 

 groove, and opercula above the follicle; c, papilla becpme a pulp, 

 and the follicle a sac, by adhesion of the opercular lips, and tho 

 secondary groove adherent, except behind the inner operculum, 

 where it has left a shut cavity of reserve for the permanent 

 tooth; d, e, temporary tooth increasing by growth downwards of 

 the fang, and the permanent tooth-sac receding from the surface; 

 /, temporary tooth appearing on the surface; g, permanent 

 tooth-sac much removed from the gum, but connected with it 

 by a cord which passes through a foramen behind the temporary 

 socket. 



56. The milk teeth are twenty in number, namely, two 

 incisors, a canine or eye tooth, and two molars, on each side 

 in the upper and the lower jaw. The permanent teeth are 

 thirty-two in number, namely : incisors and canines, replac- 



