i8o ELEMENTS OF BIOLOGY 



begins in this chamber, the sinus venosus, and propels the blood 

 into the next chamber, the atrium. From the atrium it is passed 

 into the ventricle, a compartment with thick muscular walls. The 

 wave of contraction from the sinus venosus passes to the ventricular 

 walls and their contraction drives the blood out into the distributing 

 vessels. In the Amphibia (Fig. 120), the intermediate chamber, the 

 atrium, is divided into two compartments, one receiving the blood 

 from the lungs and the other the blood from the large veins of the 

 body by way of the sinus venosus. The ventricle is a single chamber. 

 The large vessel that leaves the ventricle very shortly divides into 

 two paths, one leading to the lungs and skin and the other to the re- 

 mainder of the body. In reptiles (Fig. 121) a partition partially or 

 completely divides the ventricle into two chambers, one of which 

 propels the blood to the lungs, and the other to the remainder of 

 the body. In birds and mammals (Fig. 122) with an efficient air- 

 breathing system and the complete change from an aquatic habitat, 

 the heart has absorbed the sinus venosus into the right atrium, or 

 auricle, and is completely divided into four separate chambers, the 

 auricles and the ventricles. Technically it is more correct to speak 

 of the receiving chambers of the heart as atria. Vessels that conduct 

 blood away from the heart are called arteries; vessels containing 

 blood flowing toward the heart are veins. 



The Human Heart. The human heart (Fig. 123) is not dif- 

 ferent from that of other mammals in any important respect. The 

 large veins, vena cav.e, empty into the right atrium, having received 

 the blood from all tissues of the body, including blood from the 

 liver containing digested foods contributed by the liver mechanism, 

 and the fats that are picked up by the lacteals and collected in the 

 thoracic duct. The blood passes to the right ventricle, impelled by 

 the wave of contraction originating in a small node in the right 

 atrium that represents the remnants of the sinus venosus. The same 

 wave of contraction drives the blood out of the right ventricle 

 through the pulmonary arteries to the lungs where it passes through 



