SECT. 104.] OSSEOUS SYSTEM. J 99 



remain in communication during the whole period of the increase 

 in thickness of the hone ; so that the formation of the gaps or 

 cavities of the hone is, at least afterwards, pre-indicated by these 

 vessels, which, according to what we have said, pass from the 

 periosteum through the ossifying blastema to the bone. Besides 

 marrow-cells, vessels and a small quantity of connective tissue, 

 the bone-cavities of the periosteal layers also contain round, 

 elongated or dentated, flattened, slightly granular, cell-like bodies 

 of o - oi'" to o"02'", and more in size, with 3 to 12, and more 

 vesicular nuclei and nucleoli, which are probably to be ascribed 

 to a multiplication of the marrow-cells. 



The periosteal layers, which, according to what w T e have stated, 

 are deposited as cribriform interrupted lamellae around the osseous 

 masses formed from the cartilages, continue to be formed in the same 

 manner, as long as the bones grow, and occasion their increase in 

 thickness. At the same time, more or less essential alterations take 

 place in them, and more especially in the large cylindrical bones. 

 In these we find, that gradually, but more evidently from and after 

 birth, a large cavity becomes developed in the interior, which at 

 first is filled with foetal marrow-cells, and subsequently w T ith fully 

 developed medulla. This medullary cavity is formed in the same 

 manner as the medullary spaces described in the previous para- 

 graph, by solution of the osseous substance of the middle portion 

 of the shaft, at first only of that which has arisen from the pri- 

 mitive nucleus, soon also of that deposited by the periosteum 

 upon the former, and thus continues to enlarge in a remarkable 

 manner as long as the bone continues growing. Whence it happens, 

 that, both at the ends of the diaphyses, and at their circumference, 

 whilst new bone is continually deposited externally, that already 

 formed is uninterruptedly absorbed from within outioards. Indeed, 

 these two processes are so combined, that the bone during its 

 development is, so to speak, several times regenerated, so that, 

 for example, the humerus of the adult does not contain an atom 

 of the osseous substance of that of the new-born infant; and, 

 again, this latter contains nothing of that of the embryo of three 

 months. These relations, as well as those of the periosteal and 

 cartilage layers, to one another, will be better understood with the 

 aid of a diagram (fig. 87). If we here compare the primitive bone 

 EE with the almost fully developed one E 4 E 4 , it is seen, that in 

 the longitudinal growth of the diaphyses of the latter on both 

 sides, at the expense of the continually growing epiphysal cartilage, 

 an elongated cone of osseous substance, 1, 2, 1', 2', and 3, 4, 3', 4', 



