BY MOLECULAR COALESCENCE. 125 
its structure absolutely solid; consequently vacuities or 
interstices are an essential part of its morphology. Hence, 
one purpose answered by lacunz in common with Haversian 
canals, medullary passages, and cavities containing medulla, 
is the prevention of a degree of solidity which would be 
incompatible with its nature as a constituent of an 
organized body. It is true that some of these spaces 
contain fat, and give passage to vessels, so that they have 
a twofold use; but the others not containing these 
structures answer only to the one purpose here mentioned, 
which admits of positive and certain proof. From these 
considerations then, I hold it as a demonstrated fact that, 
at least, one use of the lacune of bone is to prevent such 
a degree of approximation of the particles of the earthy 
matter of which it is composed as would be incompatible 
with its very organization. It now remains to examine 
how far this inferred fact agrees with the general and 
intimate structure of bone. A careful comparison of the 
vacuities in different parts of a bone, and in the bones of 
different classes of animals, will show that one description 
of space insensibly merges into another, and therefore 
that there is only one cavity in a bone whatever may be 
its magnitude, and the complexity of its form, and that 
the Haversian canals, lacune, and canaliculi are merely its 
prolongations, assuming various shapes and sizes in dif- 
ferent parts of a bone, according as circumstances may 
require. The transition of the central medulla-canal m 
the shaft of a cylindrical bone into the irregular passages 
contained in its expanded ends; and the gradual diminu- — 
tion and conversion of these into the larger Haversian 
canals, is too obvious to need any further notice. But the 
transition of Haversian canals into lacune, and of these to 
canaliculi, will require the aid of the microscope, and the 
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