CHAP, v.] GROWTH OF BONE. 121 



accounted for, as well as the facility with which the rods themselves 

 may be made to separate from one another. As for the lacunae, 

 their originally granular interior seems to be gradually removed, so 

 that they become vacuities adapted for the conveyance of the 

 nutritious fluids through the compact material of the perfect bone. 



Growth of Bone. But it must not be imagined, that, when bone 

 is once deposited in a certain form, it thenceforward permanently 

 maintains its size and shape. Though a lamella be completely 

 ossified, its particles are in constant course of change, during which 

 the most important and extensive alterations of size and figure take 

 place in a slow and gradual manner. Thus the layers first deposited 

 on the inner surface of the early cancelli are pushed out by the 

 succeeding ones, and also acquire a concomitant augmentation of 

 mass; and as, in general terms, the number of lacunae in bone is 

 proportioned to its amount, the early layers most likely increase by 

 a growth and ossification of cells in their own substance, even for 

 long after they had been pushed away from the vascular surface, 

 and supplanted by the more recent ones. Thus, though bone grows 

 chiefly by layers formed in succession on its vascular surface, yet it 

 also grows in an interstitial manner after being originally deposited. 

 It is in this way only that we can explain the great expansion which 

 the primary intercellular osseous network must undergo, to form 

 that which intervenes between the Haversian systems. 



Bone when first formed, then, is disposed as an expanded surface, 

 variously and complexly involuted, and which soon becomes covered 

 with vessels. This is the foundation for its subsequent vascularity, 

 and is the source also of that active power of internal growth, 

 which has been long a theme of admiration with physiologists. 



But the expansion of bone once deposited is limited. We before 

 observed that no part of the osseous tissue was at more than a 

 certain minute distance from the vascular surface ; and that, if it 

 were so, its nutrition could not be suitably carried on. Now, if 

 more than a certain number of laminae of new bone were laid down, 

 the earlier ones would be pushed too far from the supply of blood ; 

 and hence the limitation we have spoken of. But it is necessary 

 for bone to grow much more between the commencement of ossifica- 

 tion and the adult age than this limitation appears to allow of; and 

 here we come upon an admirable provision to meet this apparent 

 difficulty. 



In the first place, a most important process of growth is conti- 

 nually going on in the cartilage, especially near the ossifying surface, 

 by the multiplication of the cells ; and, in the latter situation, by 



