DIGESTIVE SYSTEM. 3525 



or pounding, performed by the strong muscular gizzard_, 

 assisted by stones which have been swallowed, and are re- 

 tained on purpose. In other animals this trituration is 

 accomplished by hard bony parts in the stomach, as among 

 some of the Crustacea. These expedients supply the de 

 ficiencies of imperfect mastication, and enable the gastric 

 juice, and the other secretions, to become more intimately 

 mixed with the food. What, then, are the changes which 

 the food undergoes in the stomach ? 



Without attempting to give a history of the opinions 

 which have been advanced on this subject, and expressed 

 by the terms Concoction, Putrefaction, Fermentation, Disso- 

 lution, and Maceration, let us attend to the phenomena. The 

 food, after having been mixed with the saliva and gastric 

 juice, and exposed to the heat and motion of the stomach, in 

 a short time begins to exhibit changes in its mechanical pro- 

 perties. That some changes likewise take place of a che- 

 mical nature appears obvious, from the circumstance that 

 an acid is produced. This has been considered by some 

 as the phosphoric, by others as the acetic or carbonic acid. 

 This much seems to be determined that it is a volatile 

 acid ; as its effects on n'tmus paper are temporary. But the 

 mechanical effects are the most remarkable. The appear- 

 ance of the food by degrees changes, and the whole is re- 

 duced to a somewhat uniform and pultaceous mass, termed 

 cliyme. This mechanical change takes place from the sur- 

 face to the centre of the contents of the stomach ; and by 

 means of the movements of the organ, the reduced portion 

 is withdrawn as it is prepared, and passes out. at the pyloric 

 opening of the stomach. 



After all this preparation, we might expect that the nu- 

 tritious portion should now be withdrawn and enter the sys- 

 tem. But other changes are still necessary. The pylo- 

 rus opens into the duodenum* which constitutes the row. 



