HOW ANIMALS DIGEST. 



611 



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 lack- 

 ing 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 sup- 

 ply 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 



Fig. 12. Stomach op a Sheep ; 0, gullet : r, rumen, or paunch ; h. honeycomb-bag, or reticu- 

 lum; p, manyplies, or psalterium ; a, fourth stomach, or abomasum. 



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 nutri- 

 ment directly from the digestive canal. In the lower, the food as fast 

 as digested passes directly through the walls of the canal into the tis- 

 sues ; while in the lowest animals, where the digestive cavity com- 

 municates 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 after- 

 ward, the food is changed from merely dead substance to vitalized 

 organized matter, and it is now ready to form part of the living tissues. 



