When food enters the mouth of an animal it there meets with a fluid (the saliva) which is capable of converting starchy matters into sugar, thus rendering this aliment (which is presented in a state of a nature quite insoluble at the heat of the stomach) not only soluble, but nutritious. This ensalivation takes place while the food is being masticated; when in the stomach (fig. 90) it meets with the "gastric juice," a fluid capable of dissolving both fibrine and albumen (the two other chief aliments), even when coagulated; thus all kinds of food when they have been a short time in the stomach are reduced to a liquid, and so far fitted to enter the system. The stomach is a membraneous bag, communicating at the upper orifice with the mouth, and at the lower with the intestines; its internal (mucous) coat is smooth, and contains many absorbents; its outer coat is muscular, and capable of contraction, so as to roll together its contents, thus bringing every portion into contact with the gastric juice; this mass of dissolved food has received the name of "chyme." By admixture with the bile (a fluid excreted by the liver) and other juices, it separates (like curds and whey) into a liquid and partially solid matter; the liquid part is like milk, and is called "chyle," and it is taken up by absorbent vessels, by "endosmose," a peculiar power which membranes have of allowing liquids to pass through them when there is a more dense liquid on the other side (as described in the experiment at page 123, "Vegetable Kingdom,"), and, passing through the vessels called "lacteals," is mixed there with the impure blood as it is about to be returned to the heart, and from thence to the lungs, where impurities are got rid of in the form of carbonic acid. The lungs (fig. 91), in performing this office, expose the blood (contained in thousands of minute blood-vessels) to the air which enters the windpipe at every inspiration or breath, the windpipe divides into two, and these each into two more (the bronchial tubes), and so on until they are no larger than a hair; at the end of each tube is a little "air-cell," on the membraneous walls of which the vessels containing the blood are distributed as a network. The blood when it enters the lungs is of a dark purple or blackish colour, and is loaded with carbonic acid; this blood is received by the heart from the great veins, which pour it into the right auricle, this contracting, sends it into the right ventricle, which, in turn, sends it through the pulmonary arteries into the lungs; while there, it is exposed in the network of vessels which ramify on the walls of the air-cells; another process of endosmose takes place, and the carbonic acid passes through the walls of the vessels into the air-tubes, and escapes by the breath; at the same time that this carbonic acid is cast off, an equal bulk of oxygen is absorbed from the air, so that the air which enters the