50 



ELEMENTARY TISSUES. 



tissue in which the earthy matter is laid down is a membrane, 

 composed mainly of fibres and granular cells, like imperfectly 

 developed connective-tissues. Of this kind of ossification in 

 membrane, the flat bones of the skull are examples. In the 

 other, and much more common case, of which a long bone may 

 be cited as an instance, the ossification takes place in car- 

 tilage. 



In most bones ossification begins at more than one point ; 

 and from these centres of ossification, as they are called, the 

 process of deposition of calcareous matter advances in all 

 directions. Bones grow by constant development of the car- 

 tilage or membrane between these centres of ossification, until 

 by the process of calcification advancing at a quicker rate than 

 the development of the softer structures, the bone becomes im- 



FlG. 19. 



Lamellae torn off from a decalcified human parietal bone at some depth from the 

 surface, a, a lamella, showing reticular fibres ; ft, b, darker part, where several 

 lamellae are superposed ; c, c, perforating fibres. Apertures through which perfor- 

 ating fibres had passed, are seen especially ill 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). 



pregnated throughout 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 ex- 

 tremities. Increase of the length of bones, therefore, occurs at 



