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