. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 544 Comparative Anhnal Physiology ventricle, and the auricle contracts until the high pressure in the ventricle closes the auriculoventricular (AV) valves (Fig. 197). In man the ventricles first contract with the AV valves closed (isometric contraction) for sec, then during ejection of blood for sec; ventricular relaxation lasts sec, the auricles contract for sec. and are in diastole sec^^ Thus the entire sequence in the vertebrate heart consists of successive contractions of auricles an
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 544 Comparative Anhnal Physiology ventricle, and the auricle contracts until the high pressure in the ventricle closes the auriculoventricular (AV) valves (Fig. 197). In man the ventricles first contract with the AV valves closed (isometric contraction) for sec, then during ejection of blood for sec; ventricular relaxation lasts sec, the auricles contract for sec. and are in diastole sec^^ Thus the entire sequence in the vertebrate heart consists of successive contractions of auricles and ventricles and the closure of a valve whenever the pressure on the outgoing side exceeds the pressure on the incoming side. In the frog isometric systole lasts sec, and apparently some blood remains in the ventricle after each ejection.^'^^ In the amphibian three-chambered heart oxygenated and non-oxygenated blood are not separated in the ventricle but the blood returns twice to the heart for each cycle. In fishes with a single auricle and ventricle blood passes through the gill vessels before entering the aorta. A two-channel heart not only separates oxygenated from oxygen-depleted blood but also permits higher pressures in systemic arteries than are possible after the blood has passed through gill capillaries. The establishment of a low pressure respira- tory blood shunt and consequent high systemic pressure aided reptiles and homoiotherms to withstand the rigors of terrestrial life. The chambered hearts of the molluscs consist of either one or two auricles and one ventricle. The auricles are usually receiving chambers with little mus- culature, whereas the ventricle is a strong contractile organ. The systemic. Fig. 198. Diagram of dorsal view of ventricle of the heart of Limulus polyphemus, aa, anterior aortae; la, lateral aortae; os, ostia; mnc, median nerve cord (pacemaker ganglion); In, lateral nerves. From ^jfcrlson.^ heart of the cephalopod molluscs is parti
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