74 STUDENTS HISTOLOGY 



(a) In the case of endochondral bone, the commencement of 

 ossification is indicated by the enlargement of the cartilage -cells 

 and their arrangement into vertical rows. This takes place at a 

 point called the center of ossification. (?>) The- matrix between them 

 becomes calcified, (c) From the surface of the bone, which is 

 covered by a membrane corresponding to the periosteum, processes 

 extend into the cartilage, (d) The cartilage is absorbed to make 

 room for these processes through the agency of the osteoclasts 

 (see Fig. 50). (e) The absorption proceeds until it includes the 

 region of calcified cartilage. (/) The osteoblasts, which have 

 accompanied the periostea! ingrowth, arrange themselves .on the 

 surface of the spaces resulting from absorption of the cartilage, and 

 form layers of bone, (g) At the same time the osteoblasts of the 

 osteogenetic layer of the periosteum form layers of bone beneath 

 the periosteum periosteal ossification. 



The first bone formed is soft and spongy. It will be observed 

 that as ossification proceeds the whole of the cartilage will be 

 absorbed, except that at the epiphyses. In the course of the 

 growth of the individual, the network and spaces of the origi- 

 nal spongy bone undergo considerable rearrangement. Haver- 



FIG. 50. CELLS FROM RED MARROW OF RABBIT (PRUDDEX). 



A. Marrow cells proper. 



B. Giant cells. 



sian systems result from deposition of successive layers of 

 lamellae around the spaces of spongy bone, from without inward, 

 leaving a channel at the center, which is the Haversian canal. 

 The medullary cavity is produced by the absorption of the central 

 part of the bone, while new layers continue to form under the 

 periosteum. 



