DEVELOPMENT OF THE CANCELLI. 355 



no distinct walls of the lacunae can be recognized (p. 324) ; and thus 

 the formation of the whole of the true osseous tissue is accounted 

 for, together with the lacunae and pores. 



3. The cancelli are developed by absorption of more or less per- 

 fectly formed bone-substance. During the ossification of the shafts 

 of the long bones, the osseous tissue, to the thickness of 5 V to s's 

 of an inch, is compact, and without a trace of larger cavities ; being 

 composed partly of the ossified intercellular substance of the carti- 

 lage, and partly of the cells of the latter, more or less advanced in 

 their transformation into lacunae and pores. But beyond this depth, 

 cavities at first small, and, more internally, larger, are seen, which 

 appear to be eaten out of the bone, and involve both the osseous 

 tissue, the lacunas, and pores, and the still unossified portion of the 

 cartilage. How this absorption takes place is also unknown. 



Thus it appears that while the formation of bone progresses in 

 one direction, an active resorption of a part of the bone thus formed 

 follows. The cancelli thus formed are of different form, size, and 

 direction, in different bones. 



As the medullary cavities or cancelli become developed, they are 

 also filled with a soft reddish substance the fcetal medulla con- 

 sisting at first of merely a small quantity of fluid and many rounded 

 cells containing one or two nuclei, and faintly granular contents. 

 Subsequently, however, these cells become identical with those 

 already described as occurring in certain bones in the adult (p. 340); 

 and are developed in the usual way into areolar tissue, bloodvessels, 

 fat-cells, and nerves or the true marrow of the bones (p. 340), 

 The vessels are formed very rapidly, and the fat, and then the nerves, 

 afterwards; so that vessels appear in the cancelli very soon after 

 the formation of the latter. The fat-cells are few even at birth, and 

 the nerve filaments much fewer than subsequently ; the medulla 

 being now colored entirely red by the blood, and the light-reddish 

 medulla-cells. These vessels extend into the cancelli from the car- 

 tilage vessels already described (p. 352). 



Thus the bone-substance formed from cartilage alone, is merely 

 cancellated. Hence the shafts as well as the epiphyses of the long 

 bones, at first contain only cancellated or spongy tissue; the com- 

 pact bone-substance being subsequently superadded from another 

 source, as next to be described. 



