A man’s food should contain a certain amount of coarse material—cellulose, for instance, which forms the envelope of vegetable cells. And here we see a distinctive feature of the body. Protoplasm can make no use of cellulose, there is no digestive juice which will act upon it; but its presence in the alimentary canal in the form of husk and small seeds stimulates the walls by contact, and produces peristaltic movements. In Essay II., which dealt with the chemistry of the body, we said that if we were only to live upon meat we should tax our digestive apparatus severely by having to eat more proteid than we required in order to get enough carbon. The converse, however, is equally true: if we fed only upon vegetables, we should again have to over-eat. Plants most of them contain large stores of carbohydrate; potatoes and rice are rich in starch, onions in sugar; but an exclusively vegetarian diet would necessitate our consuming huge quantities, as its proteid-supply is bad. It is defective not only in amount—for sometimes, as in beans, the proportion is fairly large—but in being so indigestible that a good deal of it passes through the body unabsorbed. Both science and experience teach that we live most economically upon the carbohydrates of vegetables and the proteid of animals, and that our food is the better for being cooked. In cooking, parasitic animals which are strong enough to survive the ordeal by acid in the stomach are killed, and the food itself is made more accessible, the indigestible cellulose envelopes of the vegetable cells being burst open, and the collagen of the connective tissues converted into gelatin. No less important than the quality of our food is the quantity; and here, again, we get a good illustration of the necessity for regarding the body as a whole. A healthy man’s appetite is his best guide, and if he follows it he cannot go far wrong. People who arrogate to themselves a wisdom superior to that of Nature little know the harm they do when they force food down an unwilling throat. The cells of the alimentary canal digest and absorb what is sent to them, minding their own business, which is not to criticise the appetite. ‘Theirs not to question why; Theirs but to do and die.’ So the digestive and excretory systems embark on hard and profitless labour, and the whole body suffers. Passing on to another subject, we find that the body eats that it may work, and works that it may eat. This cycle comes naturally enough to animals which have to go and find their food, but men with a sedentary occupation have, in view of the artificial conditions under which they live, to take constitutional exercise.