254. In nourishing the tissues and stimulating the organs, the blood parts with its nutritive and stimulating constituents, and receives in return some ingredients which can no longer be usefully employed in the economy, and others which are positively injurious. An apparatus is established for its renovation and depuration; this organ is termed the lung (fig. LIX. 5), and to this organ the blood must in like manner be conveyed. Thus the blood moves in a double circle, one from the heart to the body and from the body back to the heart, termed the systemic circle; the other from the heart to the lung and from the lung back to the heart, termed the pulmonic circle. Hence in the human body the heart is double, consisting of two corresponding parts precisely the same in name, in nature, and in office; the one appropriated to the greater, or the systemic, and the other to the lesser, or the pulmonic circulation (fig. CXIV.). 255. There is a complete separation between these two portions of the heart (fig. CXIV. 6), formed by a strong muscular partition which prevents any communication between them except through the medium of vessels. 256. The heart is situated between the two lungs (fig. LIX. 2, 5), in the lower and fore part of the chest, nearly in the centre, but inclining a little to the left side. Its position is oblique (fig. LIX. 2, 5). Its basis is directed upwards, backwards, and towards the right (fig. LIX. 2); its apex is directed downwards, forwards, and towards the left, opposite to the interval between the cartilages of the fifth and sixth ribs (fig. LIX. 2). It is inclosed in a bag termed the pericardium (fig. CXV.), which consists of serous membrane. The pericardium is considerably larger than the heart, allowing abundant space for the action of the organ (fig. CXV.). One part of the pericardium forms a bag around the heart (fig. CXV.); the other part is reflected upon the heart so as to form its external covering (fig. CXV.), and is continued for a considerable distance upon the great vessels that go to and from the heart in such a manner that this bag, like all the serous membranes, constitutes a shut sac. Both that portion of the pericardium which is reflected upon the heart, and that which forms the internal surface of the bag around it, is moistened during life by a serous fluid, which, after death, is condensed into a small quantity of transparent water. That portion of the pericardium which rests on the diaphragm (fig. LXX. 1) is so firmly attached to it that it cannot be separated without laceration, and by this attachment, together with the great vessels at its base, the heart is firmly held in its situation, although in the varied movements of the body it is capable of deviating to a slight extent from the exact position here described. [Illustration: Fig. CXV. View of the heart enveloped in its pericardium, the fore part of the latter being cut open and reflected back.]