I550 



HUMAN ANATOMY. 



Fig. 1308. 



Longitudinal ground section of 

 enamel, treated with acid, showing 

 disposition of ranges of enamel-prisms 

 (/>,/>') in stripes of Schreger. Left 

 third of figure shows alternate light 

 (i) and dark {s'} bands as seen by re- 

 flected light. X 200. (Ebner.) 



absent. The interpretation of the stripes of Retzius is still a subject of dispute. The 

 brown appearance of the stripes by transmitted light only, by reflected light appear- 

 ing bluish white, disproves the assumption that they depend upon the presence of 

 pigment within the enamel. The widely accepted view of Ebner, that the stripes are 

 due to air contained in the interfascicular clefts, has been modified by Walkhoff, who 



regards the markings as due to local diminution in the 

 calcification of the enamel-prisms during certain periods 

 in the growth of the tissue when the central as well as 

 the cortical substance of a great number of columns 

 fails to take up sufficient lime salts. 



The enamel-cuticle, or 77iembrane of Nasmyth, 

 forms a continuous investment of the crown of the 

 newly erupted tooth. In the course of time it dis- 

 appears from the areas exposed to wear, but over the 

 protected surfaces it may persist during life. The 

 membrane (.009-018 mm. in thickness) is transparent 

 and remarkably resistant to the action of acids, less 

 so to alkalies, affording admirable protection to the 

 underlying enamel. After separation from the latter 

 by acids it appears structureless, or at most granular. 

 The inner surface of the membrane presents markings 

 and slight irregularities which correspond to the free 

 ends of the subjacent enamel-prisms. 



The origin of the enamel-cuticle has been much 

 discussed, and even now is not without some uncer- 

 tainty. It may be regarded as established that it rep- 

 resents the remains of part of the tissue once concerned 

 in the production of the enamel. The latter is formed, 

 as more fully described on page 1561, through the 

 agency of the epithelial cells constituting the inner 

 layer of the enamel-organ; With the completion of their task as enamel builders, 

 these cells produce a continuous cuticular envelope which persists as Nasmyth's 

 membrane, the epithelial elements of the enamel-organ, so far as they are concerned 

 in forming enamel, subsequently degenerating. The enamel-cuticle is continuous 

 with the cortical substance of the prisms, with which it agrees in optical and chemical 

 properties, — a relation which confirms the identity of origin of Nasmyth's membrane 

 and the enamel-columns. 



The Dentine. — The dentine or ivory resembles bone both in its genesis and 

 chemical composition, being a connective tissue modified by the impregnation of lime 

 salts. Dentine exceeds bone in hardness, containing a larger proportion (72 per 

 cent.) of earthy matter and a smaller amount (28 per cent. ) of organic substance. 

 When decalcified by acids, the remaining animal material retains the previous form 

 of the dentine and yields gelatin on prolonged boiling. Dentine, like bone, is formed 

 through the agency of specialized connective-tissue cells, the odontoblasts, but differs 

 from osseous tissue in the small number of these cells which become imprisoned in 

 the intercellular matrix. When this occurs, as it exceptionally does in normal human 

 dentine and more frequently in pathological conditions or in the lower animals, the 

 dentine-cells correspond to the bone-corpuscles, both being connective-tissue elements 

 lying within lymph-spaces in the calcified intercellular substance. 



Examined in dried sections under low magnification, the dentine presents a radial 

 striation composed of fine dark lines which extend from the pulp-cavity internally to 

 the enamel or the cementum externally. These dark lines are the dentinal tubules 

 filled with air, which are homologous with the lacunae and canaliculi of bone, anC 

 contain the processes of the odontoblasts. In the crown, as seen in longitudina. 

 sections, the course of the dentinal tubules is radial to the pulp-cavity ; in the root 

 their disposition is horizontal and almost parallel. The canals, however, are not 

 straight, but sigmoid, the first convexity being directed towards the root, the second 

 towards the crown. In addition to these primary curves, which are especially marked 

 in the crown, the dentinal tubules present numerous shorter secondary curves which 



