308 THE MAINTENANCE OF THE INDIVIDUAL 



ventricle, where they are attached by tendinous chords, the chordae 

 tendineae, to small muscular projections called the papillary -muscles, 

 extending from the wall of the ventricle. Back flow is prevented 

 upon contraction of the ventricle by the closing of the flaps due to 

 pressure, while a reversal of their position is prevented by the chordae 

 tendineae and the contraction of the papillary muscles. Thus the 

 blood passes from the right ventricle into the pulmonary artery, the 

 lower portion of which is guarded against back flow by three lialf- 

 moon-shaped cups, called the semilunar valves. The blood has now 

 started toward the lungs through the pulmonary artery, which is the 

 only artery carrying unoxygenated blood, to the lungs, where car- 

 bon dioxide is given off and oxygen taken in by the hemoglobin in 

 the red blood corpuscles. It then passes into one of the larger 

 pulmonary veins and so reaches the left auricle of the heart. Here 

 the process described for the right half of the heart is repeated ex- 

 cept that the left auricular- ventricular orifice is guarded by the 

 bicuspid valve, w^hile the semilunar valves on this side of the heart lie 

 in the aorta which is the outgoing artery carrying the blood about the 

 body. 



The "beating" of the heart is a more complicated story than can 

 be elaborated here. First, as the ventricles relax, blood flows from the 

 veins into the auricles and ventricles, then the two auricles contract 

 simultaneously, further dilating the two ventricles. This is followed 

 by the immediate contraction of the two ventricles. Then follows 

 a brief period of relaxation or rest during which the auricles and 

 ventricles are being filled again, after which the cycle is repeated. 

 This forces the blood from the heart in a series of spurts, accounting 

 for the type of bleeding noted when an artery is severed, and for the 

 expansion of the elastic arteries as the blood is forced out of the heart 

 into them. 



The Aortic Arches 



As the blood goes out through the pulmonary artery it is passing 

 through the embryological remains of the aortic arches. Originally 

 six in number, these paired aortic arches are of great interest to 

 students of evolution since embryological and comparative anatomical 

 studies have yielded a very striking picture of the changes in this 

 region involved in the shift of vertebrates from water to land. From 

 fishes on up to mammals only these functional aortic arches have 

 persisted, although six pairs of aortic arches are usually reckoned as 



