STRUCTURE OF THE ELEMENTARY TISSUES. 



49 



inous femur is many times smaller than the medullary cavity even of the 

 shaft of the mature bone, and, therefore, that not a trace of the original 

 cartilage can be present in the femur of the adult. Its purpose is indeed 

 purely temporary; and, after its calcification, it is gradually and entirely 

 re-absorbed as will be presently explained. 



FIG. 61. FIG. 52. 



FIG. 51. From a transverse section through part of foetal jaw near the extreme periosteum, 

 in the state of spongy bone, p, fibrous layer of periosteum : 6, osteogenetic layer of periosteum; 

 o, csteoblasts: r. osseous substance, containing many bone corpuscles. X 300. 'a^r^vi \ 



FIG. 52. Ossifying cartilage showing loops of blood-vessels. 



wnror ui ucnusieiuii, 

 (Schofield.) 



The cartilaginous rod which forms the foetal femur is sheathed in a 

 membrane termed the perichondrium, which so far resembles the perios- 

 teum described above, that it consists of two layers, in the deeper one of 

 which spheroidal cells predominate and blood-vessels abound, while the 

 outer layer consists mainly of fusiform cells which are in the mature 

 .tissue gradually transformed into fibres. Thus, the differences between 

 VOL. I. 4. 



