From the pleura near the spine arises a membrane separating the lungs and lower thorax into equal parts through the middle. It is commonly called the mediastinum, and is so well adapted to the lungs that a defect of one lung is not easily communicated to the other.[24] Certainly the lungs inhabit the middle palace of the chest, invigorate the spirits of the heart and brain, temper the heat and avert the danger of suffocation, and have lobes like the liver. They hold the heart constantly in a kind of embrace in the manner of very caressing nurses and sing a harmony of qualities by which they soothe the individual parts of the body and make them vigorous. From that part of the mediastinum which holds the middle of the lungs, a thick and hard membrane appears which completely covers the heart,[25] called in Greek PERIKARDION. This protects the heart lest it be afflicted by accidental things; and lest it lack the moistening fomentation by which its heat is moderated. It unites the forces of the heart and prevents the exhaled spirits from being dispersed by vehement motion. Here the heart establishes itself, prince of members[26] and an organ sharpened into [the shape of] a top; hollow within; continuously palpitating by its three ventricles,[27] with an auricle on each side in which life remains the longest.[28] The seat of the spirit and a small amount of blood is in the left ventricle of the heart, from which the pulmonary vein advances and enters the lungs to receive better-prepared air from them;[29] this it introduces into the ventricles of the heart lest they become unduly heated. The right ventricle contains more and very hot blood. The vena cava rises into this[30] through the middle of the spine and pours in the tinder of the vital spirit from the liver. From this the pulmonary artery belches much blood into the lungs. In the ventricle between the right and left there is tempered blood of slight quantity. From this ventricle the large artery called the aorta arises, the vehicle of the vital spirits; it is constantly agitated up and down by the contrary motions of dilatation and constriction, and finally it is divided into myriads of branches so that it revivifies the living parts in the whole body by a gentle flatus. This is the end of the account of the vital members.