HUMAN PHYSIOLOGY. 



brought in contact with atmospheric air, gives off 

 its carbonic acid gas (which is its principal 

 impurity), and absorbs oxygen, by which pro- 

 cesses it is converted into pure or arterial blood. 

 The capillaries, 6, in which the blood is arte- 

 rialised, gradually unite to form minute veins, 

 which, again, join to form larger vessels, until 

 finally the blood is collected into four vessels, 

 known as pulmonary veins (two from each lung), 

 which pour their contents into the left auricle. 

 Only one such vessel, g, is shewn in the figure, 

 because the main object of this diagram is to 

 illustrate the mode and general direction in which 

 the blood circulates, not to indicate the special 

 vessels through which it flows in different parts of 

 the body. The blood, now fitted for the various 

 purposes of nutrition, enters the left auricle, c', 

 which, by its contraction, propels it into the left 

 ventricle, i/, through the left auriculo-ventricular 

 opening. This opening, like the corresponding 

 one in the right heart, is guarded by a valve, 

 which, from its form, is termed the mitral valve, 

 and which entirely prevents the reflux of the blood. 

 The left ventricle, i/, contracts and drives its 

 contents into the large artery, a, which repre- 

 sents the aorta the great trunk which, by means 

 of its various branches (none of which are indi- 

 cated in the diagram), supplies every portion of 

 the body with pure arterial blood. From the 

 aorta and its various subdividing branches the 

 blood passes into the capillaries, e, which occur in 

 every part of the system. In these capillaries it 

 undergoes important changes, which may be con- 

 sidered as almost exactly the reverse of those 

 which occur in the pulmonary capillaries ; it parts 

 with its oxygen, becomes charged with carbonic 

 acid, and, as it leaves the capillaries and enters 

 the minute veins formed by their union, presents 

 all the characters of venous blood. The veins 

 gradually unite till they form two large trunks, 

 termed the superior and inferior vence caves, which 

 pour their contents into the right auricle the 

 point from which we started. Only one of these 

 great veins, d, is indicated in the diagram. We 

 thus perceive that there is a complete double circu- 

 lation ; that there is a lesser circulation effected 

 by the blood in its passage from the right to the 

 left heart through the lungs, and that there is 

 a great circulation effected by that fluid in its 

 passage from the left heart, through the system 

 generally, to the right heart. 



The heart is situated in very nearly the centre 

 of the cavivy of the chest. Its form is somewhat 

 conical, the lower end tapering almost to a point, 

 and directed rather forwards and to the left. This 

 lower portion alone is movable, and, at each con- 

 traction of the heart, it is tilted forwards, and 

 strikes against the walls of the chest between, in 

 man, the fifth and sixth ribs, or a little below the 

 left nipple. All the large vessels connected with 

 the heart arise from its base, and serve, from their 

 attachment to the neighbouring parts, to keep that 

 portion of it fixed. 



The substance of the heart is composed of a 

 spiral arrangement of no less than seven layers of 

 muscular fibre. When the ventricles contract, the 

 blood is propelled from them, not in a direct 

 manner, but with a sort of spiral motion, as if it 

 were really wrung out of the heart. 



But although the heart is the chief organ for 

 propelling the blood, there are other forces at 



work. When the right ventricle contracts, blood 

 is propelled into the aorta, which, however, con- 

 tained blood at the~ time. This blood is pushed 

 forwards, and the aorta dilates. When the pro- 

 pulsive power has ceased, the aorta, being a very 

 elastic tube, recovers its original calibre. In 

 doing so, it assists in forcing the blood onwards. 

 Thus by successive portions of the larger arteries 

 acting in the same manner, dilating with the 

 impulse, and regaining their size by elasticity, 

 the original mechanical force of the heart, 

 which throws blood into the aorta in a series 

 of successive jets, is converted into a uniform 

 wave-like flow, which we term the pulse. The 

 false, which beats about seventy times per minute, 

 is the change produced in the diameter and length 

 of an artery when it receives the wave of blood. 

 The tissues also exert an attractive influence on 

 the blood, drawing it forwards ; and, consequently, 

 we find that wherever we have activity of growth 

 in any part of the body, there is a determination 

 of blood to that part. We see this in the conges- 

 tion which precedes the annual growth of a 

 stag's horn. After the blood has passed through 

 the capillaries and into the veins, the power of 

 the heart has reached a minimum. The blood is 

 now forced along the veins to the heart, chiefly 

 by the pressure of the muscles. The veins are 

 provided with valves, which are so arranged as to 

 allow the blood to flow only towards the heart ; 

 and, consequently, when a muscle contracts and 

 presses on a vein, the blood is propelled forwards. 

 Thus muscular exercise assists occasionally in 

 removing venous congestions. Lastly, the move- 

 ments of respiration affect the circulation ; inspira- 

 tion, by increasing the flow of blood along the 

 great vessels to the heart ; while expiration has the 

 contrary effect 



10. RESPIRATION. 



The organs and process of respiration now 

 claim our attention. We have already stated 

 that the blood of the arteries differs in colour 

 from that of the veins, the former being of a 

 bright scarlet tint, while the latter is purplish 

 in colour. The arterial represents pure, and the 

 venous impure blood ; the change from the former 

 to the latter having taken place in the capillaries 

 which form the bond of union between the ter- 

 mination of an artery and the beginning of a 

 vein. The chemical differences between arterial 

 and venous blood are slight, except in relation 

 to the gases held in solution in these fluids. The 

 two kinds of blood differ widely in this respect, 

 there being a smaller quantity of oxygen, and a 

 greater quantity of carbonic acid in venous than 

 in arterial blood. 



The organs by which the impure and dark 

 venous blood is converted into pure, bright scarlet 

 arterial blood, fit for nourishing the various tissues 

 of the body, are the lungs, and the agent by which 

 this change is effected is the oxygen of the air we 

 breathe. In their simplest form, as they occur 

 in certain reptiles, they are mere air-sacs, existing 

 as two elastic membranous bags, having small 

 honey-comb-like depressions on their inner surface, 

 communicating with the external air by a tube 

 known as the windpipe, or trachea, which opens 

 through the larynx or organ of voice into the throat. 

 These bags are lined by a delicate, thin, and moist 



