4G4 ANATOMICAL AND PATHOLOGICAL OBSEIiVATIONS. 



There exists in every true bone a membrane or layer of 

 much greater importance, and infinitely more extended than 

 the periosteum. Between the bloodvessels and the walls of 

 the Haversian canals there is a layer of cellular substance. 

 This cellular substance is the product, its cells being the 

 descendants of the corpuscles of the cartilage or matrix in 

 which the bone was originally formed. It forms a blastema, 

 originally produced round each cartilage corpuscle by 

 development into a linear series perpendicular to the 

 ossifying surface : each of the secondary cartilage corpuscles 

 remaining as centres, or the sources of new centres of 

 nutrition of the future bone, their progeny forming the 

 cellular mass which becomes enclosed in the capsules of com- 

 pact primary bone. When these capsules have opened into 

 one another to form the Haversian canals, a process similar 

 to the mode of development of gland ducts and capillaries, 

 the cellular mass surrounds the vessels in their course, and 

 separates them from the walls of the canals. 



That this cellular layer plays an important part in the 

 economy of bone, appears probable from the prominent 

 position it holds in its development, and from the intimate 

 connection of the Haversian canals with all the morbid changes 

 of bone. Its existence, great extent, and probable powers, 

 cannot be overlooked in any question regarding the economy 

 of bone in health or disease. 



The cellular mass, just described, fills the cancelli, or en- 

 larged Haversian chambers, of foetal bones, and, in this 

 situation, has not been overlooked by former observers. In 

 adult bones, it is in the medullary cavity, cancelli, and, to a 

 certain extent, in the larger Haversian canals, replaced by fat 

 cells. 



On the surface of young and vigorous bones I have 

 observed numerous cells, flattened, elongated, and more or less 

 turgid, belonging doubtless to the system of Haversian cells. 



