252 



PRACTICAL HISTOLOGY. 



[XXII. 



...RM. 



PC 



both they gradually become more and more oblique from the centre 

 of the crown. 



Arched or curved lines incremental lines or Schreger's lines 



may sometimes be seen crossing 

 the course of the dentine tubules. 



(c.) The enamel covering the 

 dentine on the crown of the tooth; 

 somewhat brownish-coloured con- 

 centric lines may be seen in it. 



(d.) (H) The enamel consists 

 of striated prisms, hexagonal when 

 seen in transverse section (fig. 



243)- 



The dentinal tubules lie in a 



homogeneous matrix, and are 

 wavy tubes, which divide di- 

 chotomously, and give off lateral 

 branches which anastomose with 

 other lateral branches from ad- 

 jacent tubules. At the outer part 

 of the dentine are irregular inter- 

 globular spaces, which appear 

 black when they contain air (fig. 

 244). 



The crusta petrosa consists of 

 bone a thin layer composed of 

 lamellae and bone-corpuscles, but 

 no Haversian canals (fig. 244). 



FIG. 241. V.S. Tooth in Jaw. E. Enamel; O Sryftpnpfl Trmth tnlnr>f fVm 

 D Dentine; P.M. Periodontal mem- . *> 



brane; P.O. Pulp cavity ; c. Cement ; jaw oi a small mammal, e.g., a cat, 



and decalcify a short length J 

 inch of the lower jaw in chromic 

 acid and nitric acid. It will take two or three weeks to remove 

 all the bone-salts, and the decalcifying fluid must be frequently 

 renewed. The tooth is sufficiently soft to be cut when a needle 

 can be pushed into it. Make vertical sections through the whole 

 jaw and a tooth in situ. 



Stain one section in picro-carmine, another in osmic acid (twenty- 

 four hours), and mount both in Farrant's solution or in glycerine- 



B. Bone of lower jaw ; V. Vein ; a. 

 Artery ; N. Nerve. 



(a.) (L) Observe the tooth in the alveolus or depression of the 

 jaw in which it is fixed. The enamel has disappeared. The bone 

 of the jaw with its periosteum, lining the alveolus and forming 

 there the periodontal membrane (fig. 241, P.M.). 



(b.) Next the latter on the fang the cement (C), the dentine, 



