THE DIGESTIVE SYSTEM 



245 



tened; (2) the inner enamel cells, high columnar epithelium; (3) a layer of enamel 

 pulp, situated between the other layers, and consisting of stellate anastomosing 

 cells with considerable intercellular substance (Figs. 149 and 150)- A mem- 

 brane, the cuticular membrane, is first laid down between the inner enamel cells 

 and the dentine. Each of the inner enamel cells now sends out a process, Tomes' 

 process, from its inner end. The processes are separated by a considerable amount 

 of cement, and are the beginnings of the enamel prisms. Calcification now takes 

 place both in the prisms and in the cement substance, beginning in the ends nearest 



Enamel 

 Dentine | Enamel prisms 



Outer Ij^^^gl 



__ Inner J 



cells 



Enamel pulp 



Cuticle 1 



of 

 Basal memb. J 



enamel cells 



Fig. 149. — Section through Border of a Developing Tooth of a New-born Puppy. 



(Bennett.) 



the papilla. As this proceeds outward, the prisms become much elongated and 

 the cement substance reduced in amount. Further growth in thickness of 

 enamel occurs by lengthening of the enamel prisms. During the formation of 

 the enamel, the enamel pulp and the external enamel cells disappear. 



The formation of enamel in the milk teeth begins about the end of the fourth 

 month and continues until the eruption of the teeth. The extent of the enamel 

 organ is considerably greater than that of the enamel, the former covering the 

 entire tooth, both root and crown, with the exception of the base of the papilla 

 where the latter is connected with the underlying connective tissue. As the 

 tooth develops, the enamel organ disappears over the root, remaining to form 

 the enamel only over that part of the tooth which is to be subsequently exposed. 

 The function of the enamel pulp is not known. It disappears as the tooth grows. 

 It has been suggested that it may furnish nutrition or serve as an avenue through 

 which nutrition reaches the non-vascular enamel organ. It may serve as an 

 area of least resistance through which the tooth grows. 



