HEART. 678 HEAKT. body. It is a four-chambered organ, consisting responding ventricle by means of the left auric- of two upper parts, the right and left auricle-^, ulo-ventricular oritice. Each auritle liolds with- and two lower parts, the right and left ventricles, in its cavity, moderately distended, about two This article will be confined to a description of omices of fluid. The ventricles, which constitute the human heart. It is cone-shaped and flattened, the bulk of the heart, are characterized by the and in the adult lies obliquely behind the lower great thickness of their walls and their large two-thirds of the sternum, projecting consider- capacity relatively to the auricles. The cavity ably to the left. Its base is upward, and directed of each ventricle is conoidal, that of the right somewhat backward, and is situated practically being broader and shorter and a trifle more at the level of the third intercostal space. Its capacious than that of the left. Prominent mus- apex is directed to the left, downward and for- cular ridges and papilhe, the columncE caniecc, ward, and reaches the space between the fifth and arise from the inner surface of each ventricle, and sixth costal cartilages. The heart is so tilted from them the chordw tendiiiew, fibrous cords, that the margin of its right ventricle lies upon pass to the free border of the valves which close the central tendon of the diaphragm. The adult the openings between auricles and ventricles, heart is about five inches from base to apex, by There are four valves in the heart. The mitral three and one-half inches across the base, by two valve closes the left auriculo-ventricular opening, and one-half inches thick. Its weight is, in the and consists of two segments. When these are male eleven ounces, and in the female about nine closed together the blood is prevented from re- ounces, gurgitating from the left ventricle into the left The right auricle receives the venous blood auricle. Tlie trieus]jid valve, situated in the from the vena; cavie and empties it into the right riglit ventricle, closes the right auriculo-ven- ventricle. The auricle presents a central cavity tricular opening. It consists of three segments, or sinus of nearly quadrangular shape and a each triangular in shape and adherent to each other at the portion of their free -Isc.y r.mijap margins which is nearest the fibrous ring from which they arise. The largest of the segments is nearest the pulmonary artery. The semi- lunar valves are found at the aortic and pulmonary orifices of the heart. The aorta (q.v. ) springs from the left ventricle, and through it passes the blood to the arterial system. The pulmonary artery springs from the right ventricle, and through it blood •p.smxa Passes to the lungs. The closure of the flaps of the semilunar valves Lau.ap. prevents regurgitation of the blood s.pm. i"to the aorta and pulmonary artery when the heart exjiands during the diastole. These valves are well sup- plied with fibrous material and pos- sess great rigidity, so that they re- -ani.pjn. *•'*'" their shape. They open out of the ventricles and not in their cavi- ties. The heart receives its nourish- ment from blood conveyed to it by the coronary arteries during the diastole of the heart. The veins of the heart accompany the arteries and return their blood into the cavity of the right auricle. The ANTEHIOR VIEW OF THE RIGHT CHAMBERS OK THE HEART, WITH TOE GREAT llCrVCS Of the heart rUU intO ths VESSELS. ileep and superficial cardiac plex- in.r.y innominate vein; a.a., arch of aorta; v.c.s., vena cava superior; itses and also the posterior and an- r.au.ap., riglit auricular appendi.x; o.fi-a;.. orifice o( vein of Galen; au.ov.. ,.,.;' onrmmru nlerutp'! These annulusovalis; fo. or.. fossa ovalis; Ffl.f., valve of Thebe8ius;aDt.f.ra., an- 'e>>0) coronurij piej.ubtb. ^"^.'^ terior segment of tricuspid valve; v.c.i., vena cava inferior: r.«.va.. right plexuses are formed by sympathetic segment of tricuspid valve; ren.n-., section of ventricular wall; 7..sc.r., left nerve-fibres and bv tilainents from ani-tva: lea. rp.a. (La. Ip-a. p.a. coLcar. chaten. col. can ■postpm. certain cranial nerves. The heart Bubclavian vein; J.Q.a., left carotid arter.v; r.p a., right pulmonar.v arter.v; (7.a., ductusarteriosu8;7./?.a., left pulmonary arterj;p. a., pulmonary arter.v; p..im.ra., pulmonary semilunar valves; I.au.ap., left auricular appendix; and the roots of the great vessels s.p.w., small papillar.y muscle connected with septum: col. car., coIumnjB „j.p enveloped in the pericardium carneffi ; cbo.ten., chords tendineie; ant. p.m., anterior papillary muscle; ' ^ ^ • 1 1 .•iy, Acor.a., left coronary artery; posf.p.m., posterior papillary muscle. (^-V. ), a sac 01 conical sliape Avitn base below lined with serous mem- Bmall appendix somewhat resembling a dog's ear, brane (which is reflected over the heart) and and called the auricular appendix. The walls of containing a very small amount of fluid, the auricle are composed of two layers of muscu- The contraction of the heart, which occurs lar fibre and are very thin. The left auricle from 70 to 80 times a minute, begins in the auri- resembles its fellow of the right side. It re- cles. which empty their lilood into the ventricles, ceivcs the' blood from the lungs through the four The ventricles then contract, the left to empty pulmonary veins, which empty, without valves, itself through the aorta, the right to empty itself by four distinct openings. It opens into the cor- through the pulmonary artery. The mitral valve
