STRUCTURE OF BONE. 



175 



formed at a late stage of the development of bone, where the remaining 

 tissue has acquired its completest consolidation. 



292. Now, as already remarked, the simple structure just described 

 is found, not merely in the delicate plates which form the thinnest part 

 of certain bones in Man, but also in those lamellae, which form the walls 

 of the cancelli of the larger and thicker bones. Every one of these 

 lamellae repeats, in fact, the same history. The cancelli are lined by a 

 membrane derived from that of the cavity of the shaft, over which 

 blood-vessels are minutely distributed ; between these blood-vessels and 

 the osseous texture is a layer of cells ; and from the materials selected 

 and communicated by these, each lamella is nourished, through its 

 system of radiating canaliculi and nutritive centres. The cancelli, at 

 the time of their formation in the foetal bone, are entirely filled with 

 such cells ; which appear (as will be presently explained) to be the 

 descendants of the cells of the original cartilage ; but in the adult 

 bone, a large proportion of them are filled with fatty matter, which 

 they secrete into their cavities. The vessels of the cancellated struc- 

 ture at the extremities of the long bones are derived from those of the 

 medullary cavity, which is penetrated by large trunks from the exterior ; 

 and in the flat bones, they form a system of their own, connected with 

 the vessels of the exterior by several smaller trunks. 



293. The solid osseous texture which forms the cylindrical shafts of 

 the long bones, and the thick external plates of the denser flat bones, 

 is not cut off from nutritive action in the degree in which it might seem 

 to be ; for it is penetrated by a series of large canals, termed the 

 Haversian (after Clopton Havers, their discoverer), which form a net- 

 work in its interior, and which serve for the transmission of blood- 

 vessels through its substance (Fig. 48). These canals, in the long bones, 

 run for the most part in a direction parallel to 



the central cavity ; and they communicate with 

 this, with the external surface, with the cancelli, 

 and with each other, by frequent transverse 

 branches; so that the whole system forms an 

 irregular network, pervading every part of the 

 solid texture, and adapted for the establishment 

 of vascular communications throughout. The 

 diameter of the Haversian canals varies from 

 l-2500th to l-200th of an inch, or more ; their 

 average diameter may be stated at about l-500th 

 of an inch. They are lined by a membrane 

 which is continuous with that of the external 

 surface, and which carries this inwards, so to 

 speak, to form the lining membrane of the cen- 

 tral cavity, and of the cancelli ; and the cavity 

 of the tube encloses a single twig of an artery 

 or vein. Thus we may consider the whole 

 Osseous texture as enclosed in a membranous 



Fig. 48. 



1*1 i_i J 1 L i T gitudiaal section of the compact 



bag ; on which blood-vessels are minutely dis- tissue of the shaft of one of the 

 tributed, and which is so carried into the bone ^^anui; 

 by involutions and prolongations, that no part another venous canai. 

 of the latter is ever far removed from a vascular surface. 



