52 



HAND-BOOK OF PHYSIOLOGY. 



Embryonic Spongy Bone. The embryonic spongy bone, formed as 

 above described, is simply a temporary tissue occupying the place of the 

 foetal rod of cartilage, once representing the femur; and the stages 1, 2, 

 and 3 show the successive changes which occur at the centre of the shaft. 

 Periosteal bone is now deposited in successive layers beneath the perios- 

 teum, i.e., at the circumference of the shaft, exactly as described in the 



FIG. 56. Transverse section through the tibia of a fostal kitten semi-diagrammatic. X 60. P, 

 Periosteum. O, osteogenetic layer of the periosteum, showing the osteoblasts arranged side by side, 

 represented as pear-shaped black dots oh the surface of the newly-formed bone. B, the periostea! 

 bone deposited in successive layers beneath the periosteum and ensheathing E, the spongy endochon- 

 dral bone; represented as more deeply shaded. Within the trabeculae of endochonclral spongy bone 

 are seen the remains of the calcified cartilage trabeculae represented as dark wavy lines. C, the me- 

 dulla, with V, V, veins. In the lower half of the figure the endochondral spongy bone has been com- 

 pletely absorbed. (Klein and Noble Smith.) 



section on "ossification in membrane," and thus a casing of periosteal 

 bone is formed around the embryonic endochondral spongy bone: this 

 casing is thickest at the centre, where it is first formed, and thins out 

 toward each end of the shaft. The embryonic spongy bone is absorbed, 

 its trabeculae becoming gradually thinned and its meshes enlarging, and 

 finally coalescing into one great cavity the medullary cavity of the- shaft. 

 Stage 5. Absorption of the Inner Layers of the Periosteal 



