CH. V.] 



BONE 



41 



articular extremities are found capped on their surface by a thin 

 shell of compact bone, while their interior is made up of the spongy 

 or cancellous tissue. The shaft, on the other hand, is formed almost 

 entirely of a thick layer of the compact bone, and this surrounds a 

 central canal, the medullary cavity so called from its containing the 

 medulla or marrow. 



In the flat bones, as the parietal bone or the scapula, the can- 

 cellous structure (diploe) lies between two layers of the compact 

 tissue, and in the short and irregular bones, as those of the carpus 

 and tarsus, the cancellous tissue fills the interior, while a thin shell 

 of compact bone forms the outside. 



Marrow. There are two distinct varieties of marrow the red 

 and yellow. 



Red marrow is the connective tissue which occupies the spaces in 

 the cancellous tissue ; it is highly vascular, and thus maintains the 



o 



Fir.. 54. Cells of the red marrow of the guinea-pig, highly magnified, o, A large cell, the nucleus of 

 which appears to be partly divided into three by constrictions ;' ?;, a cell, the nucleus of which 

 shows an appearance of being constricted into a number of smaller nuclei ; c, a so-called giant cell 

 or myeloplaxe, with many nuclei ; d, a smaller myeloplaxe, with three nuclei ; e i, proper cells of 

 the marrow. (E. A. Schafer.) 



nutrition of the spongy bone, the interstices of which it fills. It 

 contains a few fat-cells and a large number of marrow-cells. The 

 marrow-cells are amoeboid, and resemble large leucocytes; the 

 granules of some of these cells stain readily with acid and neutral 

 dyes, but a considerable number have coarse granules which stain 

 readily with basic dyes like methylene blue. Among the cells are 

 some smaller nucleated cells of the same tint as coloured blood- 

 corpuscles. These are termed erythrdblasts. From them the coloured 

 corpuscles of the blood are developed. There are also a few large 

 cells with many nuclei, termed giant cells or myeloplaxes (fig. 54). 



Yellow marrow fills the medullary cavity of long bones, and con- 

 sists chiefly of fat-cells with numerous blood-vessels; many of its 

 cells also are the colourless marrow-cells just mentioned. 



