42 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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. 



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

 chiefly of fat cells with numerous blood-vessels. Many of its cells are in 

 every respect similar to lymphoid corpuscles. 



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

 blood-corpuscles are derived. 



The Periosteum and Nutrient Blood-Vessels. The surfaces of 

 bones, except the part covered with articular cartilage, are clothed by a 



FIG. 53. Transverse Section of Compact Bone (of humerus). Three of the Haver sum canals 

 are seen, with their concentric rings; also the lacuna, with the canaliculi extending from them across 

 the direction of the lamella. The Haversian apertures were filled with debris in grinding down 

 the section, and therefore appear black in the figure, which represents the object as viewed with 

 transmitted light. The Haversian systems are so closely packed in this section, that scarcely any 

 inteni'isal lamellae are visible. 'X 150. (Sharpey.) 



tough, fibrous membrane, the periosteum, which is closely attached to the 

 surface of the bone. Blood-vessels are distributed in this membrane, and 

 minute branches from these periosteal vessels enter the Haversian canals 



