As the blood is forced through the heart by forcible contractions of its muscular walls, it has the action of a force pump, and gives the impulse at each beat, which we call the pulse--the dilatation of the arteries throughout the system. The contraction of the auricles is quickly followed by that of the ventricles, and then a slight pause occurs; this takes place in regular rhythmical order during health. [Illustration: PLATE XX. INTERIOR OF CHEST SHOWING POSITION OF HEART AND DIAPHRAGM.] [Illustration: PLATE XXI. Heart. Right half, red. Left half, blue. Auricles, at upper end. Ventricles, at lower end. Arteries, red. Veins, blue. 1. Left carotid artery. 2. Left jugular vein. 3. Portal system. 4. Vessels of the liver. 5. Arteries of the stomach. (Caeliac axis). 6. Vessels of the large intestine. 7. Vessels of the small intestine. 8. Artery of left kidney. CIRCULATORY APPARATUS.] The action of the heart is governed and maintained by the pneumogastric nerve (tenth pair of cranial nerves); it is the inhibitory nerve of the heart, and regulates, slows, and governs its action. When the nerve is cut, the heartbeats increase rapidly, and, in fact, the organ works without control. When the nerve is unduly irritated the holdback, or inhibitory force, is increased, and the heart slows up in the same measure. The left cavities of the heart, the pulmonary veins, and the aorta, or systemic artery, contain red or florid blood, fit to circulate through the body. The right cavities of the heart, with the venae cavae, or systemic veins, the pulmonary artery, contain dark blood, which must be transmitted through the lungs for renovation. The arteries, commencing in two great trunks, the aorta and the pulmonary artery, undergo division, as in the branching of a tree. Their branches mostly come off at acute angles, and are commonly of uniform diameter in each case, but successively diminish after and in consequence of division, and in this manner gradually merge into the capillary system of blood vessels. As a general rule, the combined area of the branches is greater than that of the vessels from which they emanate, and hence the collective capacity of the arterial system is greatest at the capillary vessels. The same rule applies to the veins. The effect of the division of the arteries is to make the blood move more slowly along their branches to the capillary vessels, and the effect of the union of the branches of the veins is to accelerate the speed of the blood as it returns from the capillary vessels to the venous trunks.