STRUCTURE OF BONE. ° 61 
of areolar tissue, by thin bony partitions, which cross each 
other in every direction, forming what is called the “ cancel- 
lated” structure. These chambers or cancelli are filled with 
marrow, like the central cavity, with which they communi- 
cate. In the flat bones, moreover,—such as those of the 
head—we find that the two surfaces are composed of dense 
lates of bone, like that which forms the shaft of the long 
cmas ; but that between them there is a layer of cancellated 
structure, filled in like manner with marrow. But when we 
examine with the microscope a thin section of even the 
densest bony matter, we find it traversed by a network of 
minute canals, continuous with the central cavity. These 
canals usually run, in the shafts 
‘of long bones, in the direction “Gime & 
of their length; and are con- [Ree OSA 
ected, every here and there, by 
‘ross branches (fig. 16). They 
e termed the Haversian canals, 
after the name of their disco- 
verer, Havers.—The lining mem- 
brane of the large central cavity 
is copiously supplied with blood- 
vessels ; and this sends off pro- #2.4iN RE wi Ha 
longations into the cancelli at f 
e extremities of the bone, and "8. Matis | sarneamraee 
Sees - Sutommni rere Thus ‘ er ¥p-4 oF a zoxe Boxe. ii 
is conveyed into the in- ol apace tocingge pe is cn ae 
ior of the baie ; but no vessels seen in longitudinal sosiion i, Ba. 
tan be traced absolutely into its Joram Systems cut across, each 
texture, so that all the spaces sae f bea enc systems cut 
which lie between the Haversian surface of bens, destitate of Sone. 
canals are as destituteofvessels as “*" 5¥S*e™s- 
is healthy cartilage. These spaces are provided with nutriment 
by the following very remarkable arrangement. 
50. When we cut across the shaft of a long bone, and 
Xamine a thin section with a microscope, we of course see 
he open extremities of the Haversian canals (fig. 17, a) ; 
just as we see the cut ends of the ducts and vessels of 
food, when we make a transverse section of a stem. 
Sround each of these apertures, the bony matter is arranged 
m concentric rings, which are marked out and divided 
