Such is the evidence at present in the hands of physiologists. Naturally, physicians have endeavoured to turn it to account. Patients have been recommended to avoid animal foods which contain nucleo-proteins—to take, instead of meat and fish, eggs, milk, cheese, vegetable-albumins. Certain physicians contend that such a diet is followed by the happiest results; others, equally competent, and perhaps less biassed by “medical theory”—the most dangerous of handicaps for anyone who practises an art which must ever remain empirical—are satisfied that equally good results are obtained by excluding from the diet eggs, milk, and cheese. Physiological discoveries suggest treatment. Modern medicine is in the fullest sense applied physiology. But treatment based upon theory must be controlled by unprejudiced observation. It is possible that the gouty diathesis may be held in check in certain cases by the exclusion from the diet of certain kinds of nitrogenous food. The experience of generations has taught us that the injudicious use of such articles of diet as fruit, pastry, sugar, which do not contain nitrogen, is the main factor in inducing an attack of gout; that imperfect digestion, sluggish circulation, insufficient activity on the part of the kidneys, lead to the accumulation in tissue-spaces of the _fons et origo malorum_. Even sweetbread, which with the precision of a chemical experiment increases the production of uric acid by a healthy person, is not necessarily found unwholesome by those who are inclined to gout. It is amongst the most digestible of all meat foods, and easy digestion covers a multitude of metabolic sins. CHAPTER IX THE CIRCULATION The blood circulates in a closed system of tubes, continuous from the heart back to the heart. The walls of these vessels separate the blood from the tissues. Nowhere, except in the spleen, does it come into contact with any cells other than the lining cells of the vessels in which it flows, and the exception made by the spleen is more apparent than real. The spleen (p. 79) is a kind of sponge invested with a firm capsule. Small arteries discharge their blood into its spaces; small veins collect it. But the organ is essentially a part of the vascular system. Its spaces take the place of the capillary vessels which connect arteries with veins in other situations. [Illustration: FIG. 10.—THE HEART CUT IN THE PLANE OF ITS LONG AXIS, AND THE VESSELS WHICH OPEN INTO AND OUT OF IT. Chordæ tendineæ attach the margins of the auriculo-ventricular valves to musculi papillares which project from the inner aspect of each ventricle.]