42 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



BONE. 



The characteristic of bone is that the matrix is solidified by a deposit of 

 earthy salts, chiefly calcium phosphate, but some magnesium phosphate and 

 calcium carbonate. 



To the naked eye there appear two plans of structure in different bones, 

 and in different parts of the same bone, namely, the dense or compact, and 

 the spongy or cancellous tissue. In a longitudinal section of a long bone, 

 as the humerus, the 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 is formed almost entirely of a thick layer 

 of the compact bone which surrounds a central canal, the medullary cavity, 

 so called from its containing the medulla, or marrow. In the flat bones, the 

 parietal bone or the scapula, a layer of cancellous structure lies between 

 two layers of the compact tissue. In the short and irregular bones, as those 

 of the carpus and tarsus, the cancellous tissue alone fills the interior, while 

 a thin shell of compact bone forms the outside. 



FIG. 52. Cells of the Red Marrow of the Guinea-pig, highly magnified, a, A large 

 cell, the nucleus of which appears to be partly divided into three by constrictions; b, 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. (Schafer.) 



The Marrow. There are two distinct varieties of marrow the red and 

 the yellow. 



Red marrow is that variety which occupies the spaces in the cancellous 

 tissue; it is highly vascular, and thus maintains the nutrition of the spongy 

 bone, the interstices of which it fills. It contains a few fat cells and a large 

 number of marrow cells, many of which are undistinguishable from 

 lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. 

 Among the cells are some nucleated cells containing hemoglobin like the 

 blood corpuscles. There are also a few large cells with many nuclei, termed 

 giant cells or myeloplaxes, which are probably derived from the ordinary 

 marrow cells, figure 52. 



