Page:The New International Encyclopædia 1st ed. v. 04.djvu/864

* CIRCULATING DECIMALS. 760 CIRCULATIOlSi. That this fraction is the same as the decimal 0.3."> may be soon by division. CIRCULATING LIBRARY. Sec Libbaky. CIRCULATING MEDIUM. See Monet. CIRCULATION (Lai. <in-uhitio, cireuhir course, from circulus, circle, dim. of circus, circle). A term used in anatomy and physiol- ogy to designate the course of tlic blood through the blood-vessels. A knowlodge of the heart, arteries, capillaries, and veins (qq.v. ) is, of course, essential to a complete understanding of the subject of circula- tion; but by means of a diagram (Fig. 1), we can indicate the circula- tion of the blood in a general way. The shaded part of Fig. 1 represents the vessels carrying the impure or venous blood, which has already given i]) its oxygen to the body and taken in exchange the carbonic-acid gas. The unshaded diagram re]iresents the vessels filled with pure blood, which is freely supplied with oxygen (arterial blood). The heart is sho'n here as composed Fig. 1. Circulation of ^{. f"",'" dl'^'nhprs, of the blood: A. heart; r. which the two right ones right ventricle; V', left ven- belong to the circulation l^'l;u^ic^i?"lT!S; 'u: "f -«'"<'"« ^lood and the vena cava; e. greater cir- two left to that of ar- culation; /;, snialler circu- torial blood. Now the latlon; f. iiulinonar.v ar-, j j f jj , , body IS brought to the right auricle of the heart (c) by two large veins, the sujierior vena cava and the inferior vena cava, both of which are here represented by d. By the contraction of this chamber, the blood is forced through the right auriculo-ventricular opening into the second chamber of the riglit side of the heart, the riglit ventricle (i-), and this by its contraction drives the blood to the lungs in the direction of the arrow pointing to f. The blood ii prevented from returning into the auricle by the tricuspid valve, which completely closes the auriculo-ventricular opening during the contrac- tion of the ventricle. In its passage through the lungs the blood is purified and oxygenated, and then is brought back to the heart again by the four pulmonary veins, entering the auricle on th," left side. When this contracts, the blood is forced onward into the left vcntrielo. and then by v<'ntricular contraction into the aorta for general arterial distribution. The mitral valve prevents regurgitation into the left auricle, and the semi-lunar valve at the beginning of the aorta stops any reflux into the ventricle. (Sim- ilar valves are present in the pulmonary artery.) The aorta divides into branches, and these in turn into smaller ones, until the whole body is supplied by a minute artcri;i! plexus, or network ; the smallest arteries divide into a finer network of still more minute vessels, the capillaries, whioh have extremely delicate walls, so that the blood can come info the clo.sest relation with the cells of the body-tissues. It is in these capil- laries that the oxygen is given off, the nourish- ment furnished to the body elements, and the waste ])ioducts taken up into the blood. The capillaries then unite to form a venous plexus, and later small veins which unite with each other to form larger ones until we have all the blood collected into the superior and inferior vense cava and thus brought to the heart again. We see, from this description, that there are in reality two circulations — one. a short cir- cuit, from the right side of the heart through the lungs to the left side of the heart ; and the second, a longer circuit, from the left side of the heart through the body back to the right side of the heart. In the heart the two circula- tions connect with each other, and become con- tinuous. In addition to the puhnoitari/ and si/s- tonic circulations, described above, we have another subsidiary to the venous system, and known as the portal circuUitioii. This is not indicated in the diagram. A certain amount of the blood of the intestines is collected into the portal vein and carried to the liver, where it traverses a capillary network in intimate rela- tion with the liver-cells. Bile is formed and other important changes are ellVctod in the blood, which is liighly charged with foodstufl's recently absorbed in its passage thnnigh the intestinal capillaries. The blood is collected a second time into veins, and carried to the inferior vena cava, where it again joins the general circulation. In its passage through other sjiecial organs, the blood imdergoes further modifications. See Kid- ney: Spleen: etc. The anatomy of the organs concerned is given elsewhere, and we can consider theiii here only in their mechanical relation to the circulation. The Fig. 2. The lunge, heart, and principal blood-vessels In man; », A, veins (roni right and left arms: h, t right and left jugular veine, returuiug the blood from liead and neck. These four veina unite to" form a single trunk, the v^na. cava stiju'rior, which eiil^-rs the right auricle /; r. e. the right ancl left carotid arteries, the latt^'r rising dli-ectl.v from the arch ut the aorta. /('; the former from a sln»rt trunk called the arteria innoiiiinata ; f^. the left subclavian arter.v, rising directl.v from the aorta, while the right sub- clavian art^-r.v rises from the arteria iniiomirtata : f/. the trachea or windpijie ; /. A-. right and left lungs; /. /. the right and left auricles; /), the right ventricle; o, the ai>ex of the ventricle; m, the inferior or ascemling vena cava; D. the descending aorta, emerging from behind the lieart; q, the jiulmonary arter.v. heart is situated in the anterior part of the chest, l.ying between the riglit and left lungs, and inclosed in a membranous sac (the peri-