HISTOLOGY 



lac 



FIG. 73. -Transverse section of a Harversian sys- 

 tern of bone, h.c., Harversian canal; can., can- 

 aliculi; lac., lacunae; la., lamellae. 



Provision is also made, by fine channels called canaliculi, for the 



processes of the bone cells to 

 reach and anastomose with each 

 other, thus forming radial path- 

 ways for the transportation of 

 food and other matter from the 

 central canal, or Harversian 

 canal as it is now called, to the 

 entire group of bone cells be- 

 tween the surrounding lamellae 

 of bone. The whole structure, 

 central canal containing blood, 

 nerve and reserve bone cells, 

 together with the surrounding 

 lamellae and the bone cells 

 between them, is called an 

 Harversian system (Fig. 73). 



Bone is made up of a mass 

 of these structures formed not 

 only by the transformation or, to be more accurate, the reconstruc- 

 tion of cartilage, but also by 

 the activity of the membrane 

 surrounding the bone, the peri- 

 osteum, which is the same as the 

 perichondrium. This periosteum 

 is made up of several connective- 

 tissue layers, the inner of which 

 are modified into compact cells 

 of some size, the osteoblasts, 

 which are the same as those that 

 entered into the cartilage to re- 

 place its matrix with the bone 8 *' 

 substance. 



Where this membrane is in 

 contact with the surface of a 

 bone, such as the one whose 

 reconstruction from cartilage we 



have jUSt described, its inner layer FlG - 74- Formation of bone by the periosteum. 



. , , , . . ost., osteoblasts laying down bone and becom- 



01 OSteoblastS begin tO lay down i ng inclosed in this bone as bone cells; b, bone 



bone material (Fig. 74). Thev substance. (Part of a figure by LEWIS from 



, , ; . , , J STOHR'S Histology.) 



do not do so in simple layers 



but in long, hollow grooves whose edges are built up more rapidly than 



the rest, finally meeting and inclosing a part of the osteoblasts and a 



