. Human physiology. s semilunarvalves; 7, the arch of the aorta ; 8, the root of the pulmonary artery with its semilunarvalves ; 8, a separated portion of the pulmonary artery, remaining attached to the aortaby a cord, 9 ; 10, branches of the aorta. pressure of blood in the ventricle would act on the under surfaces of thesecusps and cause them to rise. Under these circumstances the tendinous cordsare stretched, and the edges of the cusps meet so as to stop the communica- THE HEART AND ITS DISSECTION I2? tion between the ventricle and the auricle above it. The three cusps thusform a valve calle
. Human physiology. s semilunarvalves; 7, the arch of the aorta ; 8, the root of the pulmonary artery with its semilunarvalves ; 8, a separated portion of the pulmonary artery, remaining attached to the aortaby a cord, 9 ; 10, branches of the aorta. pressure of blood in the ventricle would act on the under surfaces of thesecusps and cause them to rise. Under these circumstances the tendinous cordsare stretched, and the edges of the cusps meet so as to stop the communica- THE HEART AND ITS DISSECTION I2? tion between the ventricle and the auricle above it. The three cusps thusform a valve called the tricuspid valve ; and, from the arrangement we havejust observed, it is clear that this valve will allow a free passage of bloodfrom auricle to ventricle, but not in the opposite direction. If we now prolong the cut towards the origin of the pulmonary arteriesuntil we lay open the trunk which gives rise to these vessels, we shall findanother set of valves called the semilunar valves (Lat. semi, half; and luna,. Fig. 119.—Interior of the Bight Side of the Human Heart. 1 superior vena cava ; 2, inferior vena cava; 3, interior of the right auricle; 4, semilunar valves of the pulmonary artery ; 4, papillary muscle ; 5, 5, and 5, cusps of the tricuspidvalve; 6, pulmonary artery; 7, 8, and 9, the aorta and its branches; io, left auricle; u,left ventricle. the moon). These are three semi-circular membranous pouches, with theirconvex surfaces turned toward the ventricle. They therefore offer no obstruc-tion to the blood as it passes from the ventricle, but a backward tendency ofthe blood would fill the pouches, causing their edges to meet, and thus closingthe passage into the ventricle. Now remove the left auricle completely, with its pulmonary veins, and 128 ELEMENTARY PHYSIOLOGY notice the thinness and irregularity of its walls. We are now enabled to lookdown into the cavity of the left ventricle, which resembles the right in that amembranous valve lies between it and the aur
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