78 HANDBOOK OF PHYSIOLOGY. 



As these changes proceed, there grows up from the underlying tissue 

 into each enamel germ (fig. 78, c, p), a distinct vascular papilla (dental 

 papilla), and upon it the enamel germ becomes moulded, and presents 

 the appearance of a cap of two layers of epithelium separated by an in- 

 terval (fig. 78, c,/'). While part of the sub-epithelial tissue is elevated 

 to form the dental papillae, the part which bounds the embryonic teeth 

 forms the dental sacs (fig. 78, c, s}\ and the rudiment of the jaw, at first 

 a bony gutter in which the teeth germs lie, sends up processes forming 

 partitions between the teeth. In this way small chambers are produced 

 in which the dental sacs are contained, and thus the sockets of the teeth 

 are formed. The papilla, which is really part of the dental sac (if one 

 thinks of this as the whole of the sub-epithelial tissue surrounding the 

 enamel organ and interposed between the enamel germ and the develop- 

 ing bony jaw), is composed of nucleated cells arranged in a meshwork, 



Fig 79. Part of section of developing tooth of a young rat, showing the mode of deposition of 

 the dentine. Highly magnified, a, Outer layer of fully formed dentine; b, uncalcified matrix with 

 one or two nodules of calcareous matter near the calcified parts: c, odontoblasrs sending processes 

 into the dentine: d, pulp; e, fusiform or wedge-shape cells found between odontoblasts; /. stellate 

 cells of pulp in fibrous connective tissue. The section is stained in carmine, which colors the un- 

 calcified matrix but not the calcified part. (E. A. Schafer.) 



the outer or peripheral part being covered with a layer of columnar nu- 

 cleated cells called odontoblasts. The odontoblasts possibly form the 

 dentine, while the remainder of the papilla forms the tooth-pulp. The 

 method of the formation of the dentine from the odontoblasts is said to 

 be as follows : The cells elongate at their outer part, and these processes 

 are directly converted into the tubules of dentine (fig. 79, c], and, ac- 

 cording to some, into the contained fibrils as well. The continued for- 

 mation of dentine proceeds by the elongation of the odontoblasts, and 

 their subsequent conversion by a process of calcification into dentine tu- 

 bules. The most recently formed tubules are not immediately calcified. 

 The dentine fibrils contained in the tubules are said, by others, to be 

 formed from processes of the deeper layer of odontoblasts, which are 

 wedged in between the cells of the superficial layer (fig. 79, e} which form 

 the tubules only. There are several theories upon these points. The 

 matrix, according to more recent views, is formed by a calcification of 

 the fibrous connective tissue developed in the papilla. 



Since the papillae are to form the main portion of each tooth, i.e., the 



