Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

(1.) a clear fluid, the plasma, in which float--
(2.) a few transparent colourless bodies of indefinite and changing shape, and having a central brighter portion, the nucleus with a still brighter dot therein the nucleolus-- the white corpuscles (w.c.), and
(3.) flat round discs, without a nucleus, the red corpuscles (r.c.), greatly more numerous than the white.

Section 36. The chyle of the lacteals passes, as we have said, by the thoracic duct directly into the circulation. It enters the left vena cava superior (l.v.c.s.) near where this joins the jugular vein (ex.j.) (see Figure 1, [Sheet 2], th.d.) and goes on at once with the rest of the blood to the heart. The small veins of the villi, however, which also help suck up the soluble nutritive material, are not directly continuous with the other body veins, the systemic veins; they belong to a special system, and, running together into larger and larger branches, form the lieno gastric (l.g.v.) and mesenteric (m.v.) veins, which unite to form the portal vein (p.v.) which enters the liver (l.v.) and there breaks up again into smaller and smaller branches. The very finest ramifications of this spreading network are called the (liver) capillaries, and these again unite to form at last the hepatic vein (h.v.) which enters the vena cava inferior (v.c.i.), a median vessel, running directly to the heart. This capillary network in the liver is probably connected with changes requisite before the recently absorbed materials can enter the general blood current.

Section 37. The student has probably already heard the terms vein and artery employed. In the rabbit a vein is a vessel bringing blood towards the heart, while an artery is a vessel conducting it away. Veins are thin-walled, and therefore flabby, a conspicuous purple when full of blood, and when empty through bleeding and collapsed sometimes difficult to make out in dissection. They are formed by the union of lesser factors. The portal breaks up into lesser branches within the liver. Arteries have thick muscular and elastic walls, thick enough to prevent the blood showing through, and are therefore pale pink or white and keep their round shape.

Section 38. The heart of the rabbit is divided by partitions into four chambers: two upper thin-walled ones, the auricles (au.), and two lower ones, both, and especially the left, with very muscular walls, the ventricles (vn.). The right ventricle (r.vn.) and auricle (r.au.) communicate, and the left ventricle (l.vn.) and auricle (l.au.).

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.