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HANDBOOK OF PHYSIOLOGY. 



in different bones, and in different parts of the same bone, namely, the 

 dense QT compact, and the spongy or cancellous tissue. 



Thus, in making a longitudinal section of a long bone, as the 

 humerus or femur, 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, 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 con- 

 taining the medulla or marrow. 



In the flat bones, as the parietal bone or the scapula, one layer of the 

 cancellous structure 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 alone 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. 



Fig. 59. Cells of the red marrow of the guinea-pig, highly magnified, a, A large cell, the nu- 

 cleus 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 myelo-plaxe, with three nuclei; e-i, proper cells of 

 the marrow. (E. A. Schafer.) 



Red marrow is that variety which occupies the spaces in the cancel- 

 lous 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 undis- 

 tinguishable from lymphoid corpuscles, and has for a basis a small 

 amount of fibrous tissue. Among the cells are some nucleated cells of 

 very much the same tint as colored blood-corpuscles. There are also 

 a few large cells with many nuclei, termed giant-cells or myeloplaxes, 

 which are derived from over-growth of the ordinary marrow-cells 

 (fig. 59). 



Yellow marrow fills the medullary cavity of long bones, and consists 

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

 are in every respect similar to lymphoid corpuscles. 



