and some lie far inside the mouth where they arise from 

 pharyngeal tissues. The fine anatomy of the tooth is best 

 observed in the mammal, which is the source of the various 

 terms applied to teeth (Figure 8-49). 



Structure and origin of mammalian teeth 



The tooth is capped with a thick layer of enamel super- 

 imposed over a layer of dentine enclosing a pulp cavity. 

 The root of the tooth is embedded in a bony socket to which 

 it is connected by cementum and Sharpey's fibers (peri- 

 odontal membrane). Both the enamel and the dentine show 

 growth lines and the enamel also shows refractive lines. 



The growth lines of the enamel are called stripes or lines 

 of Retzius; these indicate periodic depositions. The enamel 

 is formed of thin prismatic rods standing nearly perpendic- 



ular to the interface membrane between the enamel and 

 the dentine. These thin rods are cemented together by 

 a calcified substance of different density. The rods of enamel 

 have the same shape as the base of the cell which produces 

 them. The spiraling of the rods as they are laid down pro- 

 duces the refractive lines of Schreger. In addition to the 

 spirals, the rods show very short kinks. The outer surface of 

 the enamel cap when completed is covered by two mem- 

 branes which wear away as soon as the tooth is used. 



The interface between enamel and dentine is richer in 

 organic material than either, and the two substances are 

 interconnected by fine fibers. Some of the dentinal tubules 

 extend a short distance into the enamel. A short distance 

 inside this interface are some irregular spaces, the inter- 

 globular spaces, the product of incomplete calcification. 

 The dentine has rather coarse tubules extending from the 



two generations 

 of cerototrichia 



neural arch 



A PRIMITIVE AGNATH 



B ARTHRODIRE 

 BOTHRIOLEPIS 



C SHARK AND 

 ACANTHODIAN 



ointed (bony) 

 ceratotrichium 

 lepidotrichium) 



proximal unjointed 

 segment 



bony radial 



D DIPTERUS 



« actinotrichium 



lepidotrichium 



ted ceratotrichium 

 lepidotrichium) 



basal extension of ray 



NEOCERATODUS 



GYROPTYCHIUS 



G EUSTHENOPTERON 



H 



TELEOST OR 

 POLYPTERUS 



Figure 8-45. Semidiogrammotic cross sections of dorsal fins to show interrelationship of parts. (After 

 Jarvik, 1959) 



TEETH 



243 



