STRUCTURE OF THE ELEMENTARY TISSUES. 43 



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, 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 

 yelloiv. 



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 undistinguishable 



FIG. 44. Cells of the red marrow of the guinea pig, highly magnified, a, a large cell, the nucleus 

 partly divided into three by constrictions; 6, a < 

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



1UC" 



marrow. (E. A. Schiifer.) 



of which appears to be partly divided into three by constrictions; 6, 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 thr^ nuclei ; e t, proper cells of the 



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" (myeloplaxes) which are derived from 

 over-growth of the ordinary marrow-cells (Fig. 44). 



Yelloio 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 show, large quantities of red blood-cor- 

 puscles. 



Periosteum and Nutrient Blood-vessels. The surfaces of bones, 

 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 part supplied with nourishment, minute 



