78 NORMAL HISTOLOGY. 



osseous tissue which was first formed in the diaphy- 

 sis is absorbed, and the medullary cavity is formed 

 in the place which it originally occupied. 



3. When bone is formed in membranes of fibrillar 

 connective tissue, as in the skull-cap, we notice, -first, 

 that some of the interlacing bundles which occupy 

 the place of the future bone become infiltrated with 

 lime salts ; along these calcareous bundles cells be- 

 come very abundant, osteoblasts appear and arrange 

 themselves, and bone forms around them just as in 

 the other varieties of bone formation. Blood-vessels 

 and marrow-tissue lie between the new-formed layers 

 of bone fc so that at a certain period of embryonic life 

 the bones of the skull-cap consist of a series of bony 

 lamellae, arranged so as to enclose branching and 

 communicating cavities, which are occupied by 

 blood-vessels and marrow-tissue, and whose' walls are 

 lined with osteoblasts. A well-defined periosteum 

 is finally formed, beneath which successive layers of 

 new osseous tissue are deposited, and thus the bone 

 increases in thickness and acquires its smooth sur- 

 faces. 



The mode of origin of the osteoblasts is still very 

 obscure. Many investigators believe that in the 

 intra-cartilaginous ossification they are the large car- 

 tilage-cells which we see at the calcification line, 

 which in contact with the blood-vessels become so 

 modified in form and function as to assume the role 

 of bone-formers. Others assert that the large carti- 



