fluid or fluids which are supplementary to the former. Thus we find digestive fluids secreted through the whole length of the food-canal.
Saliva is present in most animals as a lubricant for the food. But in those animals which chew the food there occurs another kind of saliva, a limpid fluid which aids mastication by softening the food. In mammals this has also a chemical power, changing starch to sugar. As the latter substance is heat-producing, this chemical energy is lacking in the cold-blooded vertebrates. In birds this saliva is replaced by the abundant pancreatic juice. It is most abundant in herbivores, as might be supposed from the fact that starch is a vegetable product.
Absorption.—The dissolved and chemically altered food is yet to be taken into the body, and carried wherever needed, either to supply deficiency or produce growth. At present we have to do only with the first process. In some degree the food is absorbed, as fast as digested, by blood-vessels through the whole alimentary tract. This is true particularly of the stomach. But in vertebrates absorption is
chiefly by minute tubes, called lacteals, which line the intestines. After passing through certain glands, these tubes unite to form a single tube known as the thoracic duct, which pours its contents into the veins in the neck.
In the higher invertebrates the blood-vessels take up all the nutriment directly from the digestive canal. In the lower, the food as fast as digested passes directly through the walls of the canal into the tissues; while in the lowest animals, where the digestive cavity communicates with the body cavity, the food freely bathes all parts of the structure. Simpler still, in the case of the tape-worm absorption is all the creature has to do.
Whether the process of absorption is wholly physical or partly vital is disputed. But some time during the process, or immediately afterward, the food is changed from merely dead substance to vitalized organized matter, and it is now ready to form part of the living tissues.