THE RED BONE-MARROW. 



91 



lated tissue, are filled with bone-marrow. The latter also extends within the larger 

 Haversian canals. 



Although originally only of one variety within the bones of the early skeleton, 

 the marrow in the adult consists of two kinds, the yellow and the red. Thus, within 

 the shaft of the long bones it consists of a light yellowish tissue, presenting the char- 

 acteristics of ordinary adipose tissue, while within the spaces of the cancellated tissue 

 at the ends of the same bones the marrow appears of a dull red color. In addition 

 to the ends of the long bones, the localities in which red marrow especially occurs 

 are the bodies of the vertebrae, the ribs, the sternum, the diploe of the cranium, and 

 the short bones. 



Red Marrow. — The ingrowth of the periosteal tissue and blood-vessels con- 

 stitutes the primary marrow within the embryonal skeleton ; from this tissue the red 

 marrow filling the young bones is directly derived. The red marrow is, therefore, 



Fig. 122. 





I » 



\ 



' ,11% 



I)' 



■Dense fibrous layer 



Last formed lamella- 

 of bone 



■' « 



[i 





Periosteal blood- 

 vessel passing into 

 the bone 



Bone-cell within 

 lacuna 





-Marrow-tissue continu- 

 ous with periosteum 





i 





•Remains of osteogenetic 

 layer 



'V-' 



Section of young periosteum and subjacent bone. X 275. 



the typical and first formed variety within the foetus and the young animal ; subse- 

 quently, that situated within the shaft of the long bones becomes converted into 

 yellow marrow by the replacement of the majority of the marrow elements by fat- 

 cells. 



The red marrow (Fig. 123), when examined in section after fixation and staining 

 presents a delicate reticulum of connective tissue which supports the numerous 

 medullary blood-vessels and the cellular elements. Next the bone the fibrous tissue 

 forms a thin membrane, the endosteum, lining the medullary cavity and the larger 

 Haversian canals into which the marrow extends. This membrane is highly vascu- 

 lar, its vessels joining those within the osseous canals on the one side and those of 

 the marrow on the other. 



The delicate fibrous reticulum, in addition to the thin-walled blood-channels 

 which it supports, contains within its meshes the several varieties of elements chat- 



