JG6 ANATOMY AND PHYSIOLOGY 



tinguislied 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 wjjen 

 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 copies 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,. 



