1560 



HI MAN 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 mesoclermic 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 imagination 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. 



Sublingual space 



Oral epithelium 



j_ Developing 

 ,__* - gland 



-y3g ' Dental papilla 



.';. '.'- 



Developing 

 bone of 



nianiltlile 



Muscular fibres 



Skin 



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



Tin- outer layer consists of larger and smaller epithelial cells of flattened form 

 and about .010 mm. average di.-im.-t.-r ; these cells send numerous processes into the 

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

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

 arc intimately united. 



Tli- niitM/<- layer of the t-n.im.-1-organ consists apparently of mucoid tissue, 

 Since H presents a n.-t 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 conse.iuence of an enormous detention of the intercellular 

 Spaces by tlui.l an. I a .onv.ponding elongation of the intercellular bridges, the rpi- 

 thdial plates m this manner I.ein- re.lnee.l to stellate cells connected by long delicate 

 I'"' inner bonier,,! the highly characteristic middle layer" forms a transi- 

 tion rone, known asthemfemAto/; l.iver, in which gradations from the modified to 

 tneordiimry type oi stratified epithelium art- seen. The intermediate layer is best 

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



