154 OF THE MEDULLA OR MARROW OF BONE. 



proof that the hind of tissue formed by living matter 

 depends upon its powers rather than upon its position 

 or the conditions to which it is exposed. In certain 

 forms of bone cancer very minute portions of actively 

 growing bioplasm are sometimes carried to the lungs, 

 and grow and multiply and give rise to bone cancer 

 in the pulmonaiy tissue, pi'oving that the bioplasm 

 possesses the peculiar property or power of forming 

 this particular tissue if supplied with pabulum. 



314. Of the medulla or marrow of bone. — The 

 medulla of bone is a form of almost pure adipose 

 tissue, which contains very little connective tissue, 

 associated with it. It is a question of great in- 

 terest how this adipose tissue is produced. It fills 

 the cancelli, and exists in quantity in the medullary 

 cavity, and is even found in large Haversian canals. 

 There is no doubt that the elementary parts which 

 form at length the fat cells of the marrow, are the 

 direct descendants of the bioplasts which gave origin 

 to those taking part in the formation of the bone. In 

 fact the proj^er marrow ceils (myeloid cells) may 

 become converted into bone-tissue or into marrow. 

 During development, as would be supposed, these 

 " myeloid " cells contain little or no fat, but as the 

 bone attains its permanent character, many of them, 

 become "fat cells" instead of being converted into 

 bone-tissue. In the majority of birds these cells do 

 not form fat, but as the bones are freely penetrated 

 by ail', the matter which would be fat under other 

 circumstances, is probably oxidised as fast as it is 

 produced, and eliminated as carbonic acid. 



215. Of the marrow cells, or myeloid cells, and of 

 the formation of the plates and spicules of the can- 

 cellated tissue. — The little plates or cylindrical 

 spicules of bone which enter into the formation of 

 the cancelli are represented at first by soft masses, 

 consisting of bioplasts which correspond to several 

 elementary parts of bone. These masses may often 



