THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 643 



are first produced, in the manner previously described, vascular canals, 

 which arise by the dissolution of the cartilaginous substance ; the 

 canals unite to constitute large medullary spaces, at the surfaces of 

 which osseous tissue is then secreted. 



By a slowly progressing enlargement of the bony nucleus, which 

 continues for years, the epiphysial cartilage is gradually converted 

 into a spongy osseous disc, being finally reduced to small remnants. 

 First, there is preserved, as an investment of the free surface, a layer 

 only a few millimetres thick, which constitutes the " articular 

 cartilage." Secondly, there remains for a long time a thin layer of 

 cartilage between the older, bony middle piece and the bony disc-like 

 epiphysis, and this serves to keep up the elongation of the skeletal 

 part. For the cartilage grows vigorously by the proliferation of its 

 cells, and thus is being renewed as fast as its two flat surfaces are 

 dissolved away by the endochondral ossification which takes place at 

 its expense, both by the growth of the bony epiphyses and, to a much 

 greater extent, by that of the more rapidly elongating diaphysis. 



Thus it happens that long bones which have not yet ceased 

 .growing can be divided into three pieces, if the organic parts are 

 removed by maceration. A fusion into a single osseous piece does not 

 take place until, at the time of maturity, the increase in the length 

 of the body has ceased. Then the thin plates of cartilage between 

 the diaphysis and its two epiphyses are broken down and converted 

 into bony tissue. From this time forward a further increase in the 

 length of the bone is impossible. 



Besides the three typical and chief centres already described, from 

 which the ossification of the cartilaginous fundament of a tubular 

 bone proceeds, there are established in many cases smaller centres of 

 ossification of secondary importance, which are denominated accessory 

 bone-nuclei. They always arise in the later years, when the epiphyses 

 are well developed, and sometimes not until they are in process of 

 fusion with the diaphysis. They then appear at places where the 

 cartilaginous fundament possesses elevations and projections, as in 

 the tubercles of the humerus, in the trochanters of the femur, the 

 epicondyles, etc. They serve for the conversion of these elevations 

 into osseous masses, which are generally the last to fuse with the 

 chief bone. 



After this general description, I add some detailed statements 

 about the formation and the number of the move important bony 

 nuclei in the fundaments of the separate tubular bones, concerning 

 which we have the extensive investigations of SCHWEGEL. 



