182 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



membrane. The limits of the calcifying deposit may be traced by the 

 opaque and granular character of the parts affected by it ; and it gradu- 

 ally extends itself, involving more and more of the surrounding mem- 

 brane, until the foundation is laid for the entire bone. Everywhere the 

 part most recently formed consists of a very open reticulation of fibro- 

 calcareous spicula, whilst the older part is rendered harder and more 

 compact by the increase in the number of these spicula. As the process 

 advances, and the plate of bone thickens, a series of grooves or furrows, 

 radiating from the ossifying centre, are found upon its surface ; and 

 these, by a further increase in thickness, occasioned by a deposit of 

 ossific matter all around them, are gradually converted into close canals 

 (the Haversian), which contain blood-vessels, and are lined by processes 

 of the investing membrane. The lacunae and canaliculi seem to take 

 their origin in the cells which are interspersed among the fibres, their 

 prolongations extending themselves, and insinuating themselves through 

 the spaces left between the interlacing fibres, whilst the process of cal- 

 cification is going on. 



304. The first osseous tissue which is formed by either of these pro- 

 cesses, has an irregular cancellated structure, analogous to that which 

 is found at the extremities of the long bones in adults. This is gradu- 

 ally modified by changes which essentially consist in absorption and 

 new deposition ; for the absorptive process first unites minute areolse 

 into larger ones, by removing their partitions ; and it is upon their in- 

 terior walls that new osseous lamellae are now deposited, from materials 

 supplied by the blastema they contain. It is by a process of this kind, 

 that the central medullary cavity is first formed in the bones of young 

 animals. At an early period, no such cavity exists, and its place is 

 occupied by small cancelli; this is the permanent condition of the bones 

 in most Reptiles. The cancelli gradually enlarge, however ; and those 

 within the shaft coalesce with one another until a continuous tube is 

 formed, around which the cancelli are large, open, and irregular. At 

 the same time, the diameter of the surrounding shaft is increasing by 

 the process of interstitial growth just described; so that the size of the 

 medullary cavity at last becomes greater than that of the whole shaft 

 when its formation commenced. The aggregation of the osseous matter 

 in a hollow cylinder, instead of a solid one, is the form most favourable 

 to strength, as may be easily proved upon mechanical principles. The 

 same arrangement is adopted in the arts, wherever it is desired to obtain 

 the greatest strength with a limited amount of material. 



305. The growth of Bones takes place by the addition of new tissue 

 to the part already formed ; but this addition may take place in three 

 modes, namely, by the development of new bone in the cartilage yet 

 remaining between the different centres of ossification ; by the develop- 

 ment of new bone in the membrane covering the surface; and by the 

 interstitial formation of new layers within the Haversian canals and 

 cancelli of the part already formed, by which the requisite solidity is 

 given to it. Of the first process we have the most characteristic ex- 

 ample in the increase in length of a long bone, by the ossification of 

 the cartilage which intervenes between the shaft and the epiphyses, 

 and which continues to grow, up to the time of the final union of these 



