THE SKKT.FTAI. SVST^.^r 201 



that it functionates as a place where blood cells are destroyed. It 

 is also active in the development of bone. 



Yellow marrow [¥ig. 120) consists almost wholly of fat cells, 

 which have gradually replaced the other marrow elements. Under 

 certain conditions the yellow marrow of the bones of the old or 

 greatly emaciated undergoes changes due for the most part to the 

 absorption of its fat. Such marrow becomes reddish and assumes 

 a somewhat gelatinous appearance. It is known as ^'gelatinous 

 marrow'^ Under certain conditions, e.g., fracture of shaft of long 

 bone, yellow marrow may assume the character of red marrow and 

 take an active part in the process of repair. It also serves as a 

 storage place for fat. 



The large marrow cavities, such as those of the shafts of the long 

 bones, are lined by a layer of fibrous connective tissue, the endosteiim. 



Blood-vessels. — The blood-vessels of bone pass into it from the 

 periosteum. Near the centre of the shaft of a long bone a canal passes 

 obliquely through the compact bone. This is known as the nutrient 

 canal and its external opening as the nutrient foramen. This canal 

 serves for the passage of the nutrient vessels — usually one artery and 

 two veins — to and from the medullary cavity. In its passage through 

 the compact bone the nutrient artery gives off branches to, and the 

 veins receive branches from, the vessels of the Haversian canals. 



Each of the fiat and of the short bones has one or more nutrient 

 canals for the transmission of the nutrient vessels. 



In addition to the nutrient canals the surface of the bone is every- 

 where pierced by the already mentioned (page 196) Volkmann's 

 canals, which serve for the transmission of the smaller vessels. In 

 compact bone these vessels give rise to a network of branches which 

 run in the Haversian canals. In spongy bone the network lies in 

 the marrow spaces. Branches from these vessels pass to the marrow 

 cavity, and there break up into a capillary network, which anasto- 

 moses freely with the capillaries of the branches of the nutrient artery. 



The capillaries of marrow empty into wide veins without valves, 

 the walls of which consist of a single layer of endothelium. So thin 

 are these walls that the veins of marrow were long described as pass- 

 ing over into open or incompletely walled spaces in which the blood 

 came into direct contact with the marrow elements. These veins 

 empty into larger veins, which are also valveless. Some of these con- 

 verge to form the vein or veins which accompany the nutrient artery; 

 others communicate with the veins of the Haversian canals. 



