The Circulatory System 163 



pumping mechanisms which force the blood through its circuits. The 

 left ventricle has a much heavier wall than does the right, a differ- 

 ence which is correlated with the fact that the left ventricle pumps 

 the blood through the longer systemic circuit. The right ventricle 

 pumps the blood through the much shorter pulmonary circuit. 



In the adult mammal, the right and left sides of the heart are com- 

 pletely separated; in the fetus (the unborn young) there is a passage- 

 way between the two ventricles. This opening, the joramen ovale, 

 closes soon after birth. As the lungs are not functional until birth, it 

 is not important for all the blood to pass through the pulmonary cir- 

 cuit. This opening allows a large portion of the blood to by-pass this 

 circuit. 



The auricles and ventricles of each side of the heart are separated 

 from one another by elastic flaps, the auricular-ventricular valves. The 

 valve on the right side of the heart, the tricuspid, has three flaps ; that 

 on the left, the bicuspid or mitral, has two flaps. Attached to the flaps 

 are special fibrous cords, the chordae tendineae, which in turn are con- 

 tinuous with special muscles, the papillary muscles of the ventricular 

 wall. During relaxation of the ventricles, the flaps point downward 

 and allow the passage of the blood from the auricles. When the 

 ventricles contract, the back pressure of the blood pushes against the 

 valves, forcing them shut. The flaps of the valves close tightly against 

 one another, forming a complete separation of the two chambers. The 

 flaps are prevented from bulging back into the auricle by the chordae 

 tendineae, which are kept taut by the contraction of the papillary mus- 

 cles. 



When blood leaves the ventricles and goes into either the pul- 

 monary or main systemic artery (the aorta), its backflow into the ven- 

 tricles is prevented by another series of valves. These valves are lo- 

 cated at the base of each of these arteries and consist of three half-moon- 

 shaped pockets, the semilunar valves. The edges of these valves project 

 into the lumen of the arteries and allow the free forward flow 

 of the blood. When the ventricle contracts, these flaps separate 

 and the blood passes forward ; then when the ventricle relaxes, the back 

 pressure of the blood in these pockets brings the edges together and 

 prevents any backward flow. 



The Path of the Blood Through the Heart. — Blood arriving 

 at the heart from the systemic circulation first enters the right auricle 

 through either the post- or precavat vein. This blood is low in oxy- 



