MAMMALIA 287 



supply the heart wall with blood, and the blood is returned to 

 the right side of the heart by the coronary veins. 



The beat of the heart consists in a contraction of the 

 auricles, followed immediately by the contraction of the ven- 

 tricles. The period of contraction is called the systole, and the 

 relaxation the diastole. After a short pause the auricular 

 systole and the ventricular systole take place again, and are 

 repeated, and it has to be remembered that the contractions 

 begin long before the birth of the animal and cease only with 

 its death. The arrangement of the cavities and the distri- 

 bution of the blood will be apparent. The auricular systole 

 drives the blood of the auricles into the ventricles, the right 

 receiving the venous blood from the body, and the left the 

 arterial blood from the lungs. The valves between the auricles 

 and ventricles close, and are still more firmly shut during the 

 contraction of the ventricles, thus preventing the blood return- 

 ing. The contraction of the ventricle opens the semilunar 

 valves, and the blood is discharged into the pulmonary artery 

 on the one hand and into the aorta on the other. The 

 emptying of the auricles and their relaxation is followed by a 

 flow of the blood from the caval veins and from the pulmonary 

 veins. On the other hand, the blood which is sent into the 

 aorta and the pulmonary artery immediately the systole of 

 the ventricles is finished rushes back, but it at once distends 

 the pocket valves at the base of each of these vessels. The 

 nodules, or corpora Arantii, come into apposition and the 

 edges of the valves are firmly knit together. Thus a reflected 

 wave is sent after the primary wave, and these events may be 

 read in the pulse, which indicates in the arteries the successive 

 pulsations of the ventricles. 



The circulation of the blood may be followed in the rabbit. 

 The aorta forms an arch on the left side, the corresponding 

 arch of the right side present in the embryo disappearing. 

 Its first branch is the innominate artery, which divides at 

 once into subclavian and carotid arteries, supplying the limb 

 and the neck and head on the right side. As the aorta bends 

 to the left, it yields the left carotid and the left subclavian. 

 Eeaching the dorsal part of the mediastinum it sends paired 

 vessels to the thoracic walls, and these are called intercostals. 



