320 THE TEETH. 



those, namely, which are situated between these two layers, undergo 

 remarkable changes. Originally spheroidal, they for a time merely in- 

 crease in number, but eA'entually, assuming a stellate form, they send out 

 branches which join with one another (fig. 221, c), whilst a clear jelly- 

 like matter collects in their interstices. Next, however, to the tesse- 

 lated layer is a stratum of epithelial cells, which retain their original 

 spheroidal shape. 



Into the cavity containing the enamel-germ numerous small papillary 

 processes of the vascular sac and adjacent mucous membrane project, 

 and between these, on the other hand, epithelial processes extend from 

 the enamel-germ into the membrane. The enamel-germ is now designated 

 the '' enamel organ," organon adamanUmc of Purkinje, who named the 

 columnar epithelium on the surface of the pulp the memlrana 

 adamant imc, or enamel membrane. 



The enamel prisms appear to be formed by the columnar cells 

 of the enamel-organ, either by direct calcification of their substance 

 or by deposition. It is true that in sections a space is commonly 

 observed between the cells and the newly-formed enamel but this is 

 probably produced after death by a shrinking of the soft parts. The 

 process of formation commences next to the forming dentine, almost as 

 soon indeed as the latter begins to be produced. The enamel snbstance 

 is at first soft and friable, but gets hard eventually. The enamel organ 

 extends no farther than the crown of the tooth, to which, therefore, the 

 deposit of enamel is limited. As the formation of enamel becomes 

 completed the rest of the enamel organ dwindles away : the superficial 

 tesselated layer is believed to become the "cuticula dentis." 



The cement begins to be formed simultaneously with or soon after 

 the dentine of the fang by the subperiosteal tissue, as in the formation 

 of the superficial layers of a bone. 



Eruption of the temporary Teeth. — At the time of birth the 

 crowns of the anterior milk-teeth, still enclosed in their sacs, are 

 completed within the jaw, and their fangs begin to be formed. Their 

 appearance through the gums follows a regular order, but the period at 

 which each pair of teeth is cut varies within certain limits. The 

 eruption commences at the age of seven months, and is completed 

 about the end of the second year. It begins with the central incisors 

 of the lower jaw, which are immediately followed by those of the upper 

 jaw; and, as a general rule, each of the lower range of teeth rises 

 through the gum before the corresponding tooth of the upper set. The 

 following scheme indicates, in months, the order and time of eruption 

 of the milk-teeth : — 



MOLAKS. CANINES. INCISORS. CANINES. MOLARS. 



Before the teeth, proti-ude through the gum, this imdergoes some peculiar 

 changes : its edge at first becomes dense and sharp, but, as the tooth approaches 

 it, the shaip edge disappears, the gum becomes rounded or tumid, and is of a 

 pui-plish hue ; the summit of the tooth is seen like a white spot or line through 

 the vascular gum, and soon afterwards rises through it. As the crown of the 

 tooth advances to its ultimate position, the elongated fang becomes suiTOunded 

 by a bony socket or alveolus. Before the eruption, the mucous membrane is 

 studded with a number of small white bodies, which vrere described by Seixes as 



