62 
STRUCTURE OF BONE. 
by circles of little dark spots; and when these spots are 
examined with a higher magnifying power, it is seen that 
they are small flattened cavities, from which proceed a number 
of extremely minute tubules (A). These tubules pass out 
Fig. 17.— Transverse Section of Bone. 
Showing the concentric rings round a a , the Haversian canals. At A are seen 
some of the cavities with^their radiating tubes, more highly magnified. 
from the two flat sides of each cavity; one set passes inwards, 
towards the centre of the ring, and the other outwards, to¬ 
wards the ring that next surrounds them. These minute 
tubuli, which are far smaller than the smallest blood-vessels, 
may thus be traced into every part of the substance of the 
bone; and those proceeding from different rings are so con¬ 
nected with each other, that a communication is- established 
between the innermost and the outermost circles. The tubuli 
which open upon the sides of the Haversian canals, are thus 
enabled to take up the nourishment with which they are 
