266 MICROSCOPIC A^^i<>^[Y OF THE ORGANS. 



a iv derived, on the contrary, from cells with a single nucleus 

 which has divided to form many nuclei without a correspond- 

 ing division of the protoplasm. This group of cells is made up 

 of the so-called osteoclasts, which play an important part in the 

 development of bone, and are spoken of in the discussion of this 

 subject. 



5. Eosinophiles are found often in bone-marrow ; and also, 



6. Mast-cells (/-granulations), which are found exception- 

 ally in the blood. 



Some of these marrow cells contain pigment granules, which 

 are the remains of disintegrated red blood-corpuscles. In the 

 red marrow fat cells are not abundant, and the blood-vessels 

 and nerves are found only in small number. 



The yellow or fatty marrow, which owes its color to the large 

 proportion of fat present in it, arises from the red marrow in 

 the diaphyses of the long bones by a diminution of the marrow 

 elements and an increase in fat. In old or emaciated individ- 

 uals the yellow marrow becomes reddish and resembles mucus. 

 Such a marrow is poor in fat, and is known as gelatinous bone- 

 marrow. The connective tissue, which occurs only in small 

 quantities in bone-marrow, is collected at the periphery of the 

 marrow cavity, where it forms a firm fibrous membrane, lining 

 the whole cavity. This represents a sort of inner periosteum, 

 and is called the endosteum. 



The bone, periosteum, and bone-marrow are supplied more 

 or less richly with blood-vessels. These enter the perios- 

 teum, and from here they pass, by means of the Volkmann's 

 and Haversian canals through the bone to form a network of 

 vessels in the bone-marrow. All these vessels anastomose with 

 one another. The so-called nutrient arteries, which supply 

 the medulla with blood, break up into numerous branches, 

 which form a rich capillary network in the medulla. Narrow 

 capillaries broaden out, so that in joining together they pass 

 into small veins with very delicate walls. 



The veins of the bone-marrow as well as the bone possess 

 no valves. The older idea, that the capillaries and small veins 

 possessed no wall at all, or that they were in many places 



