986 SPECIAL PHYSIOLOGY. 



ondary capsules are formed around each new cell, whilst the capsule of the 

 parent cell is fused with the intercellular matrix, and so disappears. Fig. 122, 

 c. The secondary capsules of the new cells, in their turn, blend with the ma- 

 trix and cease to be visible, whilst fresh formations of still newer cells arise 

 by division of their nuclei. The matrix thus formed is apparently homogene- 

 ous., but exhibits faint striae on the application of reagents. Sometimes a nu- 

 cleus, instead of dividing, and each new one forming a separate capsule, be- 

 comes invested by a series of concentric capsules, developed one within the 

 other. Sometimes no secondary capsules are formed, the primary one only 

 persisting. The contents of the growing cells become clearer, and the nucleus 

 plainer. Amyloid substance has been found in young cartilage cells. 



In^&ro-cartilage, the intermediate matrix, with some of the cells, becomes 

 changed into connective tissue, whilst other cells remain in the form of car- 

 tilage cells. In the yellow cartilages, intermediate elastic tissue fibres are de- 

 veloped between the cells, the rest of the matrix preserving its cartilaginous 

 character. 



Osseous Tissue and the Bones. 



The mode of formation of perfect bone is exceedingly complex. It is never 

 developed directly, but by the intervention either of cartilage, or of an imper- 

 fectly formed fibrous connective tissue, the iwecursvry tissues, as they are called, 

 in which ossification takes place. 



In the intra-cartilaginous mode of origin of bone, the cartilage cells first 

 rapidly multiply, by the formation of new cells within the old ones, giving 

 rise to clusters of softish nucleated cells in a clear matrix. In a long bone, 

 these cells are largest in the centre, and have their long diameter across the 

 axis of the bone ; as they grow, the matrix usually becomes faintly fibrillated. 

 "Wherever ossification is about to begin, there appear in the cartilage substance 

 certain soft ramified spots, which consist of masses of smaller nucleated cells, 

 out of which the bloodvessels and blood are formed ; these ramified soft spots 

 coalescing, form canals, whilst the contained bloodvessels, establishing com- 

 munications with previously existing ones, supply blood to the cartilage about 

 to undergo ossification. The cartilage cells are first grouped into elongated 

 clusters, separated by a clear matrix ; a deposition of earthy matter then 

 takes place in this matrix between the cells, in the form of opaque granules, 

 by which process the matrix becomes darker by transmitted light, and is soon 

 calcified. This calcifying process partially affects the outer walls or secondary 

 membranes of the cells, but not their contents. 



The cartilage which is thus changed into bone, is called cartilage of ossifica- 

 tion, ossifying cartilage, or precursory cartilage. The bony tissue into which it 

 is converted, is called the primary bone ; for it is no sooner formed than, as a 

 general rule, it begins to be the seat of further important changes, during 

 which it is almost entirely, or entirely, absorbed, to make way for the devel- 

 opment of the secondary bone, or perfect osseous tissue. This last alone pos- 

 sesses lacunae and canaliculi, concentric laminae, Haversian canals, and their 

 contained bloodvessels, cancelli, and in the case of the long bones, a central 

 medullary cavity, with its vascular and medullary contents. 



The change is thus accomplished. The primary bone is comparatively com- 

 pact and opaque. In it, softer spots, or areolce, appear, the secondary cavities, 

 or medullary spaces (Mailer), quite distinct from the ramified vascular canals 

 in the precursory cartilage. These spaces appear to be formed by the junc- 

 tion of the adjacent clusters of cartilage cells, now disappearing, in conse- 

 quence of the solution and absorption of the intermediate, dark, granular and 

 calcified matrix. These areolse contain, at first, a softish ossifying blastema, 

 in which there very soon appear certain fresh nucleated cells, smaller than 

 cartilage cells, and resembling medullary cells ; besides these, there are devel- 

 oped ah imperfectly formed fibrous connective tissue, and, likewise, many 

 newly evolved bloodvessels. The adjacent areolae being opened into each 

 other, by the progressive absorption of the primary bone, their vessels become 

 connected ; and in their thinned walls the secondary or perfect bone is de- 

 posited, in successive layers, by the transformation of corresponding strata of 



