358 THE TISSUES. 



bones; but that it also implies an absorption of a considerable por- 

 tion of the compact tissue at first formed. In fact, while, during 

 the growth of the bones new osseous tissue is constantly deposited 

 externally, that also which is already formed is as constantly ab- 

 sorbed in its interior. So that during its growth each bone is seve- 

 ral times regenerated ; and the humerus, for instance, of the adult, 

 does not contain an atom of the osseous tissue existing in it at 

 birth. 



The fact, however, that a thin layer of compact bone-substance is 

 formed by the periosteum on the exterior of the short bones also, 

 has been stated. Absorption and regeneration of the cancellated 

 structure occurs also in the short as well as in the long bones ; but 

 far more slowly. Hence in them (e. g. the vertebrae) we find more 

 or less still remaining, of the earlier bone-structure. It should also 

 be added that some of the Haversian vessels being enlarged, con- 

 stitute the vasa nutritia of the interior of the long bones. And 

 finally, when the long bones attain to their full length, the osseous 

 tissue of the diaphysis and of the epiphyses becomes completely 

 fused together, the disk of cartilage which hitherto intervened, now 

 disappearing entirely; and the vessels originally distributed sepa 

 rately to the shaft and the extremities, at last forming anastomoses, 

 though not very numerous, through the last formed portion of the 

 bones. If the question occurs how bone is developed externally at 

 points covered directly by tendons and ligaments, without the inter- 

 vention of periosteum (p. 846), it may be suggested that these also 

 must and do increase in size, and are also constituted of the colla- 

 genous tissue, like the blastema afforded by the periosteum. At 

 any rate, interstitial changes must be occurring in the ligaments 

 and tendons till they attain to their full development; simulta- 

 neously with which, similar changes must occur in the surface and 

 the size of the bones to which they are attached. It is only by 

 admitting interstitial changes in bone, moreover, that we can account 

 for the increase in size of their foramina, and in the length of the 

 laminae of the vertebrae, &c. 



Development of the Secondary Bones. 



Bones not previously cartilaginous occur in man, only in the 

 roof of the cranium and the face. They are called secondary, be- 

 cause their development does not commence till after that of the 

 primordial or cartilaginous cranium (p. 850). This class includes 





