View of the heart with the great vessels in connection
with it, on the left side, its chambers being laid open as in
the preceding figure. 1. The four pulmonary veins opening
into, 2. the cavity of the left auricle; 3. the cut edge of the
wall of the auricle; 4. the appendix of the auricle; 5. the
cavity of the left ventricle; 6. the cut edge of the wall of
the ventricle, the greater portion of the wall having been
removed to show the interior of the chamber; 7. valve
placed between the auricle and ventricle; 8. columnæ carneæ
terminating in tendinous threads attached to the loose
margin of the valve; 9. probe passed underneath the valve
and its tendinous threads, raising them from the wall of
the ventricle similar to a refluent current of blood; 10.
passage to 11. the aorta; 12. two of the semilunar valves
placed at the mouth of the aorta, the third having been cut
away; 13. arch of the aorta; 14. the three semilunar valves
at the commencement of the pulmonary artery seen in
action, completely closing the mouth of the vessel.
265. The walls of the left ventricle are nearly as thick again as those of the right, and its fleshy columns are much larger and stronger. From the upper and back part of this fourth chamber (fig. CXVII. 11) springs the great systemic artery, the aorta, around the mouth of which are placed three semilunar valves (fig. CXVII. 12), similar to those at the mouth of the pulmonary artery.
266. The partition which divides the two sets of chambers from each other (fig. CXIV. 6) is wholly composed of muscular fibres, and is called the septum of the heart.
267. The external surface of the heart is covered by a thin but strong membrane continued over it from the pericardium. Between this membranous covering and its fleshy substance is lodged, even when the body is reduced to the greatest degree of thinness, a quantity of fat. Immediately beneath this fat are the fleshy fibres that compose the main bulk of the organ. These fibres are arranged in a peculiar manner. The arrangement is not perceptible when the heart is examined in its natural state, but after it has been subjected to long-continued boiling, which, besides separating extraneous matters from the fibres, hardens and loosens without displacing them, the manner in which they are disposed is manifest. Just at the point where the muscular fibres that constitute the septum of the auricles are set upon those which form the septum of the ventricles, and parallel with the origin of the aorta, the heart is not muscular but tendinous. The substance called tendon, it has been shown, is often employed in the body to afford origin or insertion to muscular fibres, performing, in fact, the ordinary office of bone, and substituted for it in situations where bone would be inconvenient. From the tendinous matter just indicated most of the fibres that constitute the muscular walls of the heart take their origin. From this point the fibres proceed in different directions: those which go to form the wall of the auricles ascend; those which form the wall of the ventricles pursue an oblique course downwards, and the arrangement of the whole is such, that a general contraction of the fibres must necessarily bring all the parts of the heart towards this central tendinous point. The object and the result of this arrangement will be manifest immediately.
268. The internal surface of the chambers of the heart, in its whole extent, is lined by a fine transparent serous membrane, which renders it smooth and moist; and, like all other organs which have important functions to perform, it is plentifully supplied with blood-vessels and nerves.
269. Such is the structure of the organ that moves the blood. The artery, the tube that carries it out from the heart, is a vessel composed of three distinct layers of membrane superimposed one upon another, and intimately united by delicate cellular tissue. These layers are termed tunics or coats. The external coat (fig. CXVIII. 3), which is also called the cellular, consists of minute whitish fibres, which are dense and tough, and closely interlaced together in every direction. They form a membrane of great strength, the elasticity of which, especially in the longitudinal direction, is such that, in addition to its other names, it has received that of the elastic coat.
