42 BONE. 



condition it is calcined, zone of increase. Further towards 

 the medullary cavity, the cells become replaced by a vascular 

 prolongation of the marrow, zone of invasion, and the inter- 

 vening matrix projects bare into the medullary cavity, where 

 it soon gains a covering of osseous material. The latter is 

 elaborated by the osteoblasts, which are seen in great numbers 

 upon its surface. The honeycomb of cartilaginous matrix thus 

 left standing and shrouded in bone, constitutes the temporary 

 cancellous tissue. Resorption of this cancellous material now 

 occurs by the osteoclasts, large many-nucleated corpuscles, 

 which are found upon the surface of the network, or seated 

 in shallow depressions of their own excavation. The reduction 

 of the cancellous tissue is thus brought about. It serves the 

 temporary purpose of a fixed point for the pushing off of the 

 cartilaginous head. Note the tapering edge of the shaft 

 outside the cartilage, and the strong fibrous periosteum which 

 surrounds it and the cartilage in this region. In the recess 

 formed by the projection of the condyles, note that the surface 

 of the shaft under the periosteum presents excavations in 

 which there are osteoclasts. The osteoclasts are acting as 

 bone resorbers (resorption area). 



/Shaft of growing bone of a young mammal. T.S. Shaft (kitten), 

 (p. 8, s. 22 & 24, c. P., m. B.) (L) The two layers of the periosteum 

 are clearly defined, and the richness in corpuscular elements of 

 the osteogenetic layer is striking when compared with that 

 of the adult structure. The bone is open in texture. Instead 

 of small canals there are large spaces, and Haversian systems 

 are absent. The surface of the bone is covered by a layer 

 of osteoblasts, excepting in those localities in which osteoclasts 

 occur. Remains of marrow on the inner surface are usually 

 recognisable. (//) The osteogenetic layer of the periosteum 

 presents loose strands of connective tissue, which pass from 



