5O ELEMENTARY TISSUES. 



out with, calcareous matter, and can grow no more. In 

 the long bones the main centres of ossification are seated 

 at the middle of the shaft, and at each of the extremities. 

 Increase of the length of bones, therefore, occurs at the part 

 which intervenes between the ossifying centre in the shaft 



a r 



and that at each extremity ; while increase in thickness takes 

 place by the formation of layers of osseous tissue beneath 

 the periosteum. The former is an example of ossification 

 in cartilage ; the latter of ossification in membrane. 



Teeth. A tooth is generally described as possessing a 

 'crown, neck, and fang, or fangs. The crown is the portion 

 which projects beyond the level of the gum. The neck is 

 that constricted portion just below the crown which is 



* Fig. 19. Lamellae torn off from a decalcified human parietal bone 

 at some depth from the surface, a, a lamella, showing reticular fibres ; 

 b, b y darker part, where several lamellae are superposed ; c, c, perforating 

 fibres. Apertures through which perforating fibres had passed, are seen 

 especially in the lower part, a. a, of the figure. Magnitude as seen 

 under a power of 200, but not drawn to a scale (from a drawing by 

 Dr. Allen Thomson). 



