The negative pressure in the thorax is considerable at all times. If a manometer—a =U=-shaped tube with mercury in its loop—be connected with a cannula passed through the wall of the chest, the difference of level of the mercury in the two limbs of the =U= is a measure of the force with which the lungs are endeavouring to shrink away from the chest-wall. Even at the end of expiration the mercury in the limb next the chest stands about 6 millimetres higher than the mercury in the outer limb. During a deep inspiration the pressure in the chest falls 30 millimetres below the atmospheric pressure. Hence a problem is presented of which no completely satisfactory solution has yet been given. How comes it that lymph is not sucked into the pleural cavity? In health there is no more pleural fluid than just suffices to keep the membrane moist. The endothelial cells which cover the surface of the pleura resist further exudation. Valves in the lymphatic vessels prevent backward flow. Yet in disease, when the pleura is inflamed, lymph pours out quickly, often to be reabsorbed with equal rapidity when the pleurisy subsides. This flow uphill, from a lower to a higher pressure, can be explained only as a phenomenon due to the “secretory” capacity of endothelium. As an answer to the hydrostatic problem this is hardly satisfactory. The circulation of the blood is the result of the difference between the pressure in the vessels through which it leaves the heart, and that in the vessels through which it is returned. The pressure in the aorta amounts to about 200 millimetres of mercury. In the venæ cavæ it is nil, or, owing to the aspiration of the thorax, less than nil. =The Heart.=—Inspection of the liver, the spleen, or the kidney helps but little to the comprehension of the mechanism of these organs. It is quite otherwise in the case of the heart. Its mechanics being comparatively simple, physiology is concerned with measurements, with the conditions under which it can and cannot work, and with the action upon it of the nervous system and of drugs. The heart of any mammal will suffice for anatomical study. A sheep’s heart is about the same size as that of a man, and exactly similar, save in minute particulars, which do not appreciably affect its mode of working.