BY MOLECULAR COALESCENCE. 129 
arches is completed, the membrane on which this process 
has taken place becomes completely covered with a layer 
of bone; and thus is formed the outermost circle of an 
Haversian canal, consisting externally of a layer of mem- 
brane, and internally of one of bone. Within the latter 
another layer of membrane is formed, which becomes 
coated in the same manner by a layer of bone, and thus 
the process is repeated until the Haversian canal is com- 
pleted. Now this arrangement of alternate layers of 
bone and membrane can be seen by polarized light 
distinctly in thin transverse sections preserved in Canada 
balsam or glycerme, by polarized light, the osseous portion 
appearing white, and the membranous, not polarizing the 
light, black. A difference of structure, as first pointed 
out by Tomes and De Morgan, can be distinguished also 
by the ordinary illumination, one layer presenting a more 
granular appearance than the other. These lamella are 
seen distinctly only in the higher mammalians. They are 
scarcely recognisable in the bones of cetaceans, and in 
those of birds they are absent altogether. A similar 
irregularity occurs also in respect to the lacune, canaliculi, 
and Haversian canals. The lacune and the canaliculi 
connected with them are subject to every possible 
degree of variety in different classes of animals. In 
the higher mammals the form of the lacunze is some- 
what elliptical, and the canaliculi proceeding from 
them take chiefly two directions, namely, towards the 
centre, and towards the circumference of the Haversian 
system to which the lacune belong. In birds, especially 
the high flying ones, as the condor and albatross, the 
lacunee are generally very small and of no particular form, 
and the canaliculi, which are also very minute, diverge 
from them in all directions, and, communicating very 
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