The Essentials of Digestion
A summary of the principal chemical adventures of food during its passage through the digestive tube is shown in Figure 271. The blood stream receives the food from the small intestine after it has been reduced to absorbable liquid form. This reduction, although partly mechanical, most of all involves a chemical breakdown into less complex substances, a process actuated by enzymes.
The irregular lines in the diagram indicate the particular glands from which each enzyme arises, and, at the wavy interval on the line, the general locality in the digestive tract where it is effective, and also the kind of food involved in each instance.
In conclusion it may be observed that when a hungry person sits down to a dinner table and surveys the viands spread before him, he rarely takes thought of all the necessary preliminary work that has made the occasion possible. Producers, handlers of raw materials, and an army of middlemen, as well as the manipulations of a cook, must at least be taken into account, yet in spite of all the detailed processes of preparation that have been involved, they are not a circumstance to what happens to the dinner in the twenty-five feet or so that it travels after it has been swallowed.