STRUCTURE OP BONE. 
61 
of areolar tissue, by tbin 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 
plates of bone, like that which, forms the shaft of the long 
bones; 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 
of their length; and are con¬ 
nected, every here and there, by 
cross branches (fig. 16). They 
are 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¬ 
longations into the cancelli at 
the extremities of the bone, and Fig * ^.-diagram representing 
into the Haversian canals. Ihus the shaft of a long bone. 
blood is conveyed into the in- ® transverse^ectionf'L/c,"surface 
terior of the bone; but no vessels 
can be traced absolutely into its 
texture, so that all the spaces 
which lie between the Haversian 
canals are as destitute of vessels as 
is healthy cartilage. These spaces are provided with nutriment 
by the following very remarkable arrangement. 
50. When we eut across the shaft of a long bone, and 
examine a thin section with a microscope, we of course see 
the open extremities of the Haversian canals (fig. 17, a); 
just as we *see the cut ends of the ducts and vessels of 
wood, when we make a transverse section of a stem. 
Around each of these apertures, the bony matter is arranged 
in concentric rings, which are marked out and divided 
seen in longitudinal section ; i, Ha¬ 
versian systems cut across, each 
having an Haversian canal in its 
centre; g v, Haversian systems cut 
longitudinally ; l , lamellae near the 
surface of bone, destitute of Haver¬ 
sian systems. 
