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Section 39. The blood coming from all parts of the body, partly robbed of its oxygen and containing much carbon dioxide and other katastases, enters the right auricle of the heart through three great veins, the median vena cava inferior from the posterior parts of the body, and the paired venae cavae superiores from the anterior. With the beating of the heart, described below, it is forced into the right ventricle and from there through the pulmonary artery (p.a.) seen in the figure passing under the loop of the aorta (ao.) to the lungs.

Section 40. The lungs (lg. Figure 1, Sheet 1) are moulded to the shape of the thoracic cavity and heart; they communicate with the pharynx by the trachea (tr. in Figure 1, Sheet 1) or windpipe, and are made up of a tissue of continually branching and diminishing air-tubes, which end at last in small air-sacs, the alveoli. The final branches of the pulmonary arteries, the lung capillaries, lie in the walls of these air-sacs, and are separated from the air by an extremely thin membrane through which the oxygen diffuses into, and the carbon dioxide escapes from, the blood.

Section 41. The mechanism of respiration will be understood by reference to Figure 3, Sheet 2. It will be noted, in dissecting that the lungs have shrunk away from the walls of the thorax; this collapse occurs directly an aperture is made in the thorax wall, and is in part due to their extreme elasticity. In life the cavity of the thorax forms an air-tight box, between which and the lungs is a slight space, the pleural cavity (pl.c.) lined by a moist membrane, which is also reflected, over the lungs. The thorax wall is muscular and bony, and resists the atmospheric pressure on its outer side, so that the lungs before this is cut through are kept distended to the size of the thoracic cavity by the pressure of the air within them. In inspiration (or breathing-in) the ribs are raised by the external intercostal (Anglice, between-ribs, e.i.c.m.) and other allied muscles, and the diaphragm (dia.) contracts and becomes flatter; the air is consequently sucked, in as the lungs follow the movement of the thorax wall. In expiration the intercostals and diaphragm relax and allow the elastic recoil of the lungs to come into play. The thoracic wall is simultaneously depressed by the muscles of the abdominal area, the diaphragm thrust forwards, as the result of the displacement and compression of the alimentary viscera thus brought about. (r.r.r. in the Figure mark ribs.)

Section 42. The oxygen and carbon dioxide are not carried in exactly the same way by the blood. The student will know from his chemical reading that neither of these gases is very soluble, but carbon dioxide is sufficiently so in an alkaline fluid to be conveyed by the liquid plasma. The oxygen however, needs a special portative mechanism in the colouring matter of the red corpuscles, the haemoglobin, with which it combines weakly to form oxy-haemoglobin of a bright red colour, and decomposing easily in the capillaries (the finest vessels between the arteries and veins), to release the oxygen again. The same compound occurs in all true vertebrata, and in the blood-fluid of the worm; in the crayfish a similar substance, haemocyanin, which when oxygenated is blue, and when deoxydized colourless, discharges the same function.

Section 43. The blood returns from the lungs to the left auricle (l.au.) by the pulmonary veins, hidden in the Figure by the heart, passes thence to the thick-walled left ventricle (l.vn.), and on into the aorta (ao.).

Section 44. The beating of the heart is, of course, a succession of contractions and expansions of its muscular wall. The contraction, or systole, commences at the base of the venae cavae and passes to the auricles, driving the blood before it into the ventricles, which then contract sharply and drive it on into the aorta or pulmonary artery; a pause and then a dilatation, the diastole follows. The flow of the blood is determined in one direction by the various valves of the heart. No valves occur in the opening of the superior cavae but an imperfect one, the Eustachian valve, protects the inferior cava; the direction of the heart's contraction prevents any excessive back-flow into the veins, and the onward, tendency is encouraged by the suck of the diastole of the ventricles. Between the left ventricle and auricle is a valve made up of two flaps of skin, the mitral valve, the edges of the flaps being connected with the walls of the ventricle through the intermediation of small muscular threads, the chordae tendinae, which stretch across its cavity to little muscular pillars, the papillary muscles; these attachments prevent the mitral valve from flapping back into the auricle, and as the blood flows into and accumulates in the ventricle it gets behind the flaps of the valve and presses its edges together. When the systole of the ventricle occurs, the increased, tension of the blood only closes the aperture the tighter, and the current passes on into the aorta, where we find three watch-pocket valves, with the pocket turned away from the heart, which are also closed and tightened by any attempt at regurgitation (back-flow). A similar process occurs on the right side of the heart, but here, instead of a mitral valve of two flaps between auricle and ventricle, we have a tricuspid valve with three. The thickness of the muscular walls, in view of the lesser distance through which it has to force the blood, -are- [is] less for the right ventricle than the left.

Section 45. The following are the chief branches of the aorta. The student should be able to follow them with certainty in dissection; they are all displayed in the Figure; but it must not be imagined for a moment that familiarity with this diagram will obviate the necessity for the practical work; (in.) is the innominate artery; it forks into (s.cl.a.) the right subclavian, and (r.c.c.) the right common carotid. Each carotid splits at the angle of the jaw into an internal and an external branch. The left common carotid, (l.c.c.) arises from the base of the innominate,* (l.s.cl.a.) the left subclavian, directly from the aorta. The aorta now curves round to the dorsal middle line, and runs down as seen in Figure 1, Sheet 1 (d.ao.) and Figure 1, Sheet 2 (d.ao.). Small branches are given off to the ribs, and then comes the median coeliac (coe.a.) to the stomach and spleen, the median superior mesenteric (s.mes.a.) to the main portion of the intestine, and the inferior mesenteric (p.m.a.) to