252. The blood, being necessary to nourish the tissues and to stimulate the organs, must be in motion in order to be borne to them. An apparatus is provided partly for the purpose of originating an impelling force to put the blood in motion, and partly for the purpose of conveying the blood when in motion to the different parts of the body.
253. The heart is the impelling organ; the great vessels in immediate connexion with it are the transmitting organs (fig. CXIV. 1, 2). The heart is divided into two sets of chambers (fig. CXIV. 3, 4, 10, 11), one for the reception of the blood from the different parts of the body (fig. CXIV. 3, 10); the other for the communication of the impulse which keeps the blood in motion (fig. CXIV. 4, 11). The chamber which receives the blood is termed an auricle (fig. CXIV. 3, 10), and is connected with a vessel termed a vein (fig. CXIV. 1, 2, 9); that which communicates impulse to the blood is termed a ventricle (fig. CXIV. 4, 11), and is connected with a vessel termed an artery (fig. CXIV. 7, 12). The vein carries blood to the auricle; the auricle transmits it to the ventricle; the ventricle propels it into the artery; the artery, carrying it out from the ventricle, ultimately sends it again into the vein, the vein returns it to the auricle, the auricle to the ventricle, the ventricle to the artery, and thus the blood is constantly moving in a circle; hence the name of the process, the circulation of the blood.
View of the heart with its several chambers exposed, and
the great vessels in connection with them. 1. The superior
vena cava; 2. the inferior vena cava; 3. the chamber called
the right auricle; 4. the chamber called the right ventricle;
5. the line marking the passage between the two chambers,
and the points of attachment of one margin of the valve;
6. the septum between the two ventricles; 7. the pulmonary
artery arising from the right ventricle, and dividing at 8, into
right and left for the corresponding lungs; 9. the four pulmonary
veins bringing the blood from the lungs into 10,
the left auricle; 11. the left ventricle; 12. the aorta arising
from the left ventricle, and passing down behind the heart
to distribute blood, by its divisions and subdivisions, to
every part of the body.
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.
View of the heart enveloped in its pericardium, the fore part
of the latter being cut open and reflected back.
257. When the interior of the heart is laid open there are brought into view four chambers (fig. CXIV. 3, 4, 10, 11), two for each circle. Those belonging to the pulmonic circle are on the right (fig. CXIV. 3, 4), those to the systemic on the left side of the body (fig. CXIV. 10, 11); hence the terms right and left are applied to these respective parts of the heart.