96 STUDIES IN ADVANCED PHYSIOLOGY. 



the cartilage is removed and nothing but this mesh-work of 

 masonry left, there is produced in the center of the humerus 

 what is familiar to us as spongy bone. Be it observed, 

 however, that this spongy bone is in no sense derived from 

 the cartilage, but is an entirely new product, and that all 

 the cartilage, together with its cartilage cells, has been 

 removed. While these changes are going on in the shaft 

 of the bone the osteoblasts under the periosteum have begun 

 to secrete bone there, and so have surrounded the cartilag- 

 inous shaft with a fine casing of osseous tissue. Layer 

 after layer of bony lamellae is added until there has been 

 formed a cylinder of bone of quite appreciable thickness, 

 moulded right over the original cartilage. In this way the 

 new bone receives at once the shape intended for it. While 

 the strength of the shaft is thus added to under the peii- 

 osteum, new osteoclasts, or myelo-plaques begin to remove 

 the spongy bone just deposited where the cartilage was 

 removed. In this way by the continued absorbing action 

 of these cells there is soon formed an empty space, the 

 beginning of the medullary cavity. The tunneling of the 

 cartilage extends further and further towards the ends of 

 the bone. These tunnels are then immediately lined with 

 layers of bone by succeeding osteoblasts, and so the forma- 

 tion of spongy bone proceeds gradually towards the extrem- 

 ities. This spongy bone is, however, at the rear end being 

 continually absorbed by other osteoclasts, and so the medul- 

 lary cavity reaches further and further towards the opposite 

 ends. If this continued uninterrupted and with the rapidity 

 with which it goes on for a while, it would soon remove all 

 of the cartilage, even to the very ends. But this stage is 

 never reached, for while the cartilage is being continually 

 encroached upon and removed at one end it keeps elongat- 

 ing above, continuing "to do so until the full length of the 

 adult bone has been reached. As the cartilages at the ends 

 grow the periosteum creeps further and further along it and 

 begins the deposition of bone. In this way the bony shaft 

 follows pari passu the enlargement of the cartilage. Thus 



