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 0.05 sec, 

 then during ejection of blood for 0.22 sec; ventricular relaxation lasts 0.53 

 sec, the auricles contract for 0.11 sec. and are in diastole 0.69 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 0.06-0.13 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 particularly well developed; blood re- 

 turns from the body by veins to the branchial hearts at the base of the gills 

 and passes from the gills in veins to the auricles of the systemic heart. This 

 arrangement QL"booster" hearts proximal to the gills is more efficient than 

 is the plan ir^nshes, in which the heart has to overcome the resistance of 

 vessels in both gills and body tissues. The muscle of molluscan hearts, par- 

 ticularly of cephalopods and gastropods, is striated.*^*' 



Tubular Hearts. The systemic hearts of most arthropods consist of con- 

 tractile tubes. There may be a thin receiving chamber (atrium) surrounding 

 part of the heart, as in Limulus, or the heart may be free in a large sinus. 

 It is usually anchored at several corners and receives blood through several 

 paired ostia which are valved (Fig. 198). In many insects the heart is sus- 

 pended by variously arranged alary muscles which maintain tension on the 

 heart (see Wigglesworth,^*^^ for series of types). Blood leaves the heart by 

 one or several arteries, always at the anterior end, sometimes also laterally 

 and posteriorly. Myographic records indicate that the entire tubular heart 



