THE STRUCTURE OF THE ELEMENTARY TISSUES. 45 



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 undistin- 

 guishable 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. 51). 



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 



FIG. 51. 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 ; 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 three nuclei ; e *, 

 proper cells of the marrow. (E. A. Schafer.) 



derived, as we shall presently show, large quantities of red blood- 

 corpuscles. 



Periosteum and Nutrient Blood-vessels. The surfaces of the 

 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 branches from the peri osteal vessels entering the little fora- 

 mina on the surface of the bone, and finding their way to the Haver- 

 sian canals, to be immediately described. The long bones are supplied 

 also by a proper nutrient artery which, entering at some part of the 

 shaft so as to reach the medullary canal, breaks up into branches for the 

 supply of the marrow, from which again small vessels are distributed to 

 the interior of the bone. Other small blood-vessels pierce the articular 

 extremities for the supply of the cancellous tissue. 



Microscopic Structure of Bone. Notwithstanding the differences of 



