STRUCTURE OF THE TEETH. 



301 



The dentinal tubules, as well as the fibres of Tomes, anastomose throughout 

 their entire extent by means of fine processes. As the fibres approach the enamel, 

 which they do not penetrate, some of them bend on themselves, and form a loop 

 (Fig. 124, c), whilst others pass into the " inter globular spaces" which are so 

 abundant in the outer part of the dentine (Czermak, 1850). These interglobular 

 spaces are small spaces bounded by curved surfaces. Certain curved lines 

 " Schreger's lines'' (1800), may be detected with the naked eye in the dentine (e.g., 

 of the elephant's tusk) running parallel with the contour of the tooth. They are 

 caused by the fact that at these parts all the chief curves in the dentinal tubules 

 follow a similar course (Retzius, 1837). 



The enamel, the hardest substance in the body (resembling apatite), covers 

 the crown of the teeth. It consists of hexagonal flattened prisms (Malpighi, 1687) 

 arranged side by side like a palisade (Fig. 124, a and B). They are 3-5 n (-5-^7 inch) 

 broad, not quite uniform in thickness, curved slightly in different directions, and 



c/ 



Fig. 124. 



Section of a tooth between the dentine and enamel a, enamel; c, dentinal tubules; 

 B, enamel prisms highly magnified. 



owing to inequalities of thickness, they exhibit transverse markings. They are 

 elongated, calcified, cylindrical, epithelial cells derived from the dental papilla. 



Retzius described dark-brown lines running parallel with the outer boundary of 

 the enamel, due to the presence of pigment. The fully-formed enamel is nega- 

 tively doubly refractive and uniaxial, while the developing enamel is positively 

 doubly refractive (Hoppe-Seyler). 



The cuticula or Nasmyth's membrane (1839) covers the free surface of the 

 enamel as a completely structureless membrane 1 - 2 /m thick, but in quite young 

 teeth it exhibits an epithelial structure, and is derived from the outer epithelial 

 layer of the enamel-organ. 



The cement (John Hunter, 1778) or crusta petrosa, is a thin layer of 

 bone covering the fang (Fig. 125, a). The bone lacunae communicate directly 

 with the dentinal tubules of the fang. Haversian canals and lamellae are only 

 found where the layer of cement is thick, and the former may communicate 

 with the pulp-cavity (Salter). Very thin layers of cement may be devoid of 

 bone-corpuscles. Sharpey's fibres occur in the cement of the dog's tooth (Wai- 

 deyer); while in the horse's tooth single bone -corpuscles are enveloped by a 

 capsule (Gerber). In the periodontal membrane y which is just the periosteum 

 of the alveolus, coils of blood-vessels similar to the renal glomeruli occur. 

 They anastomose with each other, and are surrounded with a delicate capsule 

 of connective-tissue (C. Wedl). 



