52 



HANDBOOK OF PHYSIOLOGY. 



purpose is indeed purely temporary; and, after its calcification, it is 

 gradually and entirely absorbed as will be presently explained. 



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

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

 osteum 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 

 the foetal perichondrium and the periosteum of the adult are such as 

 usually exist between the embryonic and mature forms of connective 

 tissue. 



FIG. 61. 



FIG. 62. 



FIG. 61. Transverse section of a portion of raetacarpal bone of a foatus, showing 1 , fibrous 

 layer of periosteum; 2, osteogenetic layer of ditto; 3 periosteal bone; 4, cartilage with matrix grad- 

 ually becoming calcified, as at 5, with cells in primary areolee; beyond 5 the calcified matrix is 

 being entirely replaced by spongy bone, x 2 '0. (V. D. Harris.) 



FIG. 62. A small isolated mass of bone next the periosteum of the lower jaw of human foetus. 

 a, osteogenetic layer of periosteum. G, multinuclear giant cells, the one on the left acting here 

 probably like an osteoclast. Above c, the osteoblasts are seen to become surrounded by an osse- 

 ous matrix. (Klein and Noble Smith.) 



Between the hyaline cartilage of which the fcetal femur consists and 

 the bony tissue forming the adult femur, two intermediate stages exist 

 viz., calcified cartilage, and embryonic spongy bone. These tissues, 

 which successively occupy the place of the foetal cartilage, are in suc- 

 cession entirely absorbed, and their place taken by true bone. 



