CH. V.] 



MARROW. 



59 



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 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 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 areolar tissue. Among the cells arc 



fiS^yr.^PV, :: '.''-/Q^^M 



Fig. 8>. 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 snows an appearance of being constricted into a number of smaller 

 nuclei ; c, a so-called giant cell, or myeloplaxe, with many nuclei ; </, a smaller myelo- 

 plaxe, with three nuclei ; i, proper cells of the marrow. (E. A. Schafer.) 



some nucleated cells of the same tint as coloured blood-corpuscles. 

 There are also a few large cells with many nuclei, termed giant 

 cells or myeloplaxes (fig. 2). 



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. 



From these marrow-cells, especially those of the red marrow, 

 are derived, as we shall presently see, large quantities of red 

 blood-corpuscles. 



Periosteum and Nutrient Blood-vessels. The surfaces of 

 l*ones, except the part covered with articular cartilage, are 

 clothed by a tough, fibrous membrane, the periosteum ; and it is 

 from the blood-vessels which are distributed in this membrane, 

 that the bones, especially their more compact tissue, are in great 



