224 STRUCTUEE OP THE HEART. 



of which the upper one is termed the auricle, and the lower 

 the ventricle. Thus we have the right and left auricles, and 

 the right and left ventricles. Each auricle communicates 

 with its corresponding ventricle, by an aperture in the 



Superior Pulm. Pulmonary 



vena cava art. Aorta artery 



7 Pulmonary veins 

 Pulmonary veins 



Right auricle (^^BT /HHW- Left auricle 



Tricuspid valves - ~ SftlS ' Mitral valve 



Inferior vena cava ; ,^^ 



/ x'HHffiSlw / "^*-.. 



^ ^^ Left ventricle 



Right ventricle 



Partition Aorta 

 Fig. 123. IDEAL SECTION op THE HUMAN HEART. 



transverse partition, which is guarded by a valve. The walls 

 of the ventricles are much thicker than those of the auricles ; 

 and for this evident reason, that the ventricles have to 

 propel the blood, by their contraction, through a system of 

 remote vessels ; whilst the auricles have only to transmit the 

 fluid that has been poured into them by the veins, into the 

 ventricles, which dilate themselves to receive it. The difference 

 in the thickness of the walls of the left and the right ventricles 

 is explainable on the same principle ; for the left ventricle 

 has to send the blood, by its contractile power, through the 

 remotest parts of the body; whilst the right has only to 

 transmit it through the lungs, which, being much nearer, 

 require a far less amount of force for the circulation of the 

 blood through them. 



258. The arterial system of the greater circulation entirely 

 springs from one large trunk, which is called the aorta (see 

 figs. 122-124); this originates in the left ventricle, and is 

 the only vessel which passes forth from that cavity. It first 

 ascends towards the bottom of the neck ; then forms what is 

 termed the arch, a sudden curve, which gives it a downward 



