352 THE TISSUES. 



middle of foetal life, preceding by a longer or shorter time the ap- 

 pearance of the centres of ossification, and accompanying the latter 

 as they increase. They may still be seen in the epiphyses of the 

 long bones in a person even eighteen years old. They always lie 

 in wide canals (55$ to 3^ of an inch even in a foetus of five months) 

 excavated in the cartilage, and bounded by narrow, elongated car- 

 tilage cells. They enter the cartilage from the perichondrium at 

 first, and, after an osseous centre exists, from the border of the 

 latter also ; proceeding in straight lines in various directions, and 

 giving off a few branches, which seem not to anastomose at all, but 

 to terminate in blind, club-shaped dilatations. These canals are 

 produced by an absorption of the elements of the original cartilage- 

 substance, and they originally contain a plastic material (cartilage- 

 marrow) composed of minute rounded cells, from which true blood- 

 vessels are developed. Of the vessels themselves, sometimes but 

 one large one ; frequently two, distinctly arterial, with muscular 

 walls ; again, onlyVapillaries in various numbers are found in a 

 single canal. It is not precisely understood how the circulation is 

 carried on by them. Their object is, doubtless, to afford a greater 

 amount of nutritive material, both for the changes in the cartilage 

 preparatory to ossification, and for the development of the bone 

 itself. That they are merely an accidental production, as H. Meyer 

 maintains, is highly improbable. 



II. Together with the formation of vessels in the cartilages, the 

 elements of the latter undergo important changes. The cartilage- 

 cavities, before of small size, and containing but few cells, begin to 

 grow, and successive generations of cells to be produced in them in 

 the following manner: Each cell is first divided into two by seg- 

 mentation transverse to the line of ossific advance; these are again 

 subdivided, and the process repeated till long lines of cartilage-cells 

 extend in the elongated cartilage-cavities from the border where 

 ossification has taken place. 1 The size of the cavities, however, 

 does not increase after birth. When the ossification of the cartilage 

 proceeds in one direction only, they are grouped in rows at the 

 border of the cartilage, in which the long cavities are being de-' 

 veloped. This is best seen in the extremities of the shafts of the 

 larger long bones; the rows of cavities being arranged in parallel 

 lines, -close together, and of considerable length, as shown by Fig. 225. 



1 Tomes and De Morgan. 



