1560 



HUMAN ANATOMY. 



After the entire dentine has been formed, the odontoblasts remain as the periph- 

 erally situated pulp-cells, retaining their connection with the dentine by means of the 

 dentinal fibres. The other portions of the dental papilla become converted into the 

 pulp-tissue, which retains the embryonal type throughout life and later receives the 

 larger vascular and nervous trunks. 



The Enamel-Organ. — The extremity of the cylinder of ectoblastic epithelium 

 which early marks the position of the future tooth by its ingrowth from the dental 

 ledge soon broadens out and becomes invaginated to form the young enamel-organ 

 overlying the apex of the mesodermic dental papilla. In contrast to the latter, which 

 as the pulp-tissue remains as a permanent structure, the enamel-organ is but embry- 

 onal and transient, and later entirely disappears. When fully developed, the enamel- 

 organ consists of three principal parts, — the outer, middle, and inner layers. Since 

 the organ, originally pyriform, is converted into a cap by the invagination of its 

 broader and deeper surface, it follows that the external and internal layers are 

 directly continuous at the margin of the inverted area. 



Fig. 1319. 



Oral epithelium 



Fibres of 

 genio-glossus 



Developing 

 bone of 

 mandible 



Developing 



gland 

 Dental papilla 



Muscular fibres 



Sagittal section through mandible and surrounding structures of eighteen-weeks foetus. X 30. 



The Older layer consists of larger and smaller epithelial cells of flattened form 

 and about .010 mm. average diameter ; these cells send numerous processes into the 

 surrounding vascular connective tissue forming the tooth-sac which invests the dental 

 germ, whereby, in conjunction with the vascular tufts, the sac and the enamel-organ 

 are intimately united. 



The middle layer of the enamel-organ consists apparently of mucoid tissue, 

 since it presents a net-work of stellate cells separated by large clear spaces. Critical 

 examination, however, shows that this tissue consists of epithelial elements which 

 have become modified in consequence of an enormous distention of the intercellular 

 spaces by fluid and a corresponding elongation of the intercellular bridges, the epi- 

 thelial plates in this manner being reduced to stellate cells connected by long, delicate 

 proces.ses. The inner border of the highly characteristic middle layer forms a transi- 

 tion zone, known as the intermediate layer, in which gradations from the modified to 

 the ordinary type of stratified epithelium are seen. The intermediate layer is best 

 marked over the upper part of the crown, at the sides thinning out and entirely dis- 



