256 



THE MECHANICS OF THE HEART 



known as the conus arteriosus. All these different parts of the heart 

 possess contractile powers, their action being coordinated in such a 

 manner that the sinus contracts first, the auricle next and the ventricle 

 and conus last of all. The blood traverses the chambers of the heart 

 in the same direction. An oscillatory flow is made impossible by : (a) 

 the proper sequence of contraction of the different segments of the 

 heart and (6) the fact that the cardiac orifices are guarded by valves 

 which open only in the direction from sinus to ventricle. 



In accordance with the force which the different parts of the heart 

 must develop in order to propel the blood, the ventricle contains a 



^^^ 



much greater amount of muscle tissue than 

 the auricle or sinus. It must be remem- 

 bered that the ventricle produces the pres- 

 sure which is necessary to drive the blood 

 through the entire vascular system. In ac- 

 complishing this end it must overcome the 

 relatively high resistance prevaihng in the 

 peripheral blood-vessels. The sinus and 

 auricle, on the other hand, pump the blood 

 merely into the adjoining ventricle and, as 

 this transfer is effected at a time when the 

 latter is in a condition of rest, the ante- 

 chambers need not develop anything more 

 than very moderate degrees of pressure. 



A peculiar modification of the circula- 

 tory system is found in fish. Inasmuch 

 as the respiratory interchange in these 

 animals is effected by means of the gills, 

 this particular circuit is most highly de- 

 veloped, while the lungs with their pulmo- 

 nary system of blood-vessels are, of course, 

 absent. The circulation of the gills is 

 made possible by a number of afferent 

 branches which are given off from the ven- 

 tral aorta and lead to the different gill- 

 plates. From here the blood is conveyed 

 to the dorsal aorta by way of the efferent vessels. In this way, only a 

 part of the blood discharged by the heart finds its way into the gills, 

 where it is aerated and is distributed subsequently to all parts of the 

 body. It is returned to the heart thoroughly charged with carbon 

 dioxid. The fourth subclass of the fishes, the Dipnoi, present rather 

 complicated conditions, because they are equipped with lungs as 

 well as with gills and hence, are in possession of a pulmonary and a 

 gill-circuit. 



The heart of the amphibians is situated in the fore part of the body 

 ventrally to the first vertebrae, and consists of a sinus venosus, two 

 auricles, and a ventricle with its bulbus arteriosus. The blood which 



Fig. 125. — Diagram to Show 

 THE Course of the Blood 

 Through the Amphibian Heart. 



SV, sinus venosus; RA, 

 right auricle; LA, left auricle; 

 V, ventricle; BA, bulbus arte- 

 riosus; A, aorta; PA, pulmo- 

 nary arteries; PV, pulmonary- 

 veins. The striated portion 

 contains venous blood, the 

 dotted portion mixed blood, 

 and the clear space, arterial 

 blood. 



