First, The saliva; secreted from the glands of the mouth:--alkaline, glairy, adhesive. Second, The gastric juice; secreted in the inner or third lining of the stomach,--an acid, and powerful enough to dissolve all the fiber and albumen of flesh food. Third, The pancreatic juice; secreted by the pancreas, which you know in animals as sweetbreads. This juice has a peculiar influence upon fats, which remain unchanged by saliva and gastric juice; and not until dissolved by pancreatic juice, and made into what chemists call an _emulsion_, can they be absorbed into the system. Fourth, The bile; which no physiologist as yet thoroughly understands. We know its action, but hardly _why_ it acts. It is a necessity, however; for if by disease the supply be cut off, an animal emaciates and soon dies. Fifth, The intestinal juice; which has some properties like saliva, and is the last product of the digestive forces. A meal, then, in its passage downward is first diluted and increased in bulk by a watery fluid which prepares all the starchy portion for absorption. Then comes a still more profuse fluid, dissolving all the meaty part. Then the fat is attended to by the stream of pancreatic juice, and at the same time the bile pours upon it, doing its own work in its own mysterious way; and last of all, lest any process should have been imperfect, the long canal sends out a juice having some of the properties of all. Thus each day's requirements call for PINTS. Of saliva 3-3/4 gastric juice 12 bile 3-3/4 pancreatic juice 1-1/2 intestinal juice 1/2 ------- 21-1/2 Do not fancy this is all wasted or lost. Very far from it: for the whole process seems to be a second circulation, as it were; and, while the blood is moving in its wonderful passage through veins and arteries, another circulation as wonderful, an endless current going its unceasing round so long as life lasts, is also taking place. But without food the first would become impossible; and the quality of food, and its proper digestion, mean good or bad blood as the case may be. We must follow our mouthful of food, and see how this action takes place.