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ANATOMY AND PHYSIOLOGY 
tinguished Physiologists maintain that the fatty mass 
is analogous to the liver; but its structure, alter- 
nate increase and decrease, and the presence else- 
where of what may, in certain cases, be considered 
as true biliary vessels, appear sufficient reasons for 
rejecting this opinion. 
Certain secretions having a direct influence on di- 
gestion have already claimed our notice while con- 
sidering that function ; but there are others, which 
may be regarded as the produce of digestion, since 
extracted from the blood, to which we have yet to 
refer ; and their importance will be judged of when 
it is mentioned, that it is almost entirely from them 
that we derive all the insect products which we have 
converted to our own use. Of these Silk may well 
be regarded as the most valuable, since it has become 
nearly as essential to our own purposes, as it is to 
the economy of the animals which produce it. The 
vessels which secrete it resemble the biliary ves- 
sels in shape, but are usually much larger, (in the 
silk- worm they are about a foot in length,) consist- 
ing of two tubes, which unite at the extremity, and 
open into a small perforated filiform organ, com- 
monly placed between the palpi on the under lip. 
This is named the spinneret, and the size of its aper- 
ture determines the thickness of the thread. The 
fluid, before it comes in contact with the air, is vis- 
cous and transparent in young larvae, but thick and 
opaque in mature ones. It is found by chemical 
analysis to be chiefly composed of a gummy matter, 
a small portion of another substance resembling wax. 
