32 CIRCULATION OF THE BLOOD. 



Describe the action of the auricles. 



During the period of rest of the heart blood flows freely from 

 the veins into the ventricles, the auriculo-ventricular valves offer- 

 ing no resistance ; but the influx is so strong that by the time the 

 heart begins to contract the auricle is quite filled and the ventricle 

 partially. The contraction of the auricle is sudden and very quick, 

 commencing at the great veins and extending toward the ventricu- 

 lar opening. Both auricles contract simultaneously. 



Why does not the auricle throw the blood back into veins ? 



(1) The power of the auricular contraction is not sufficient to 

 cause a reflux. 



(2) The muscular coat of the great veins near the heart con- 

 tracts, and helps to prevent this regurgitation. 



(3) The weight of the incoming blood opposes. 



(4) Valves in the veins oppose, and the Eustachian valve guards 

 the inferior vena cava. 



Describe the action of the ventricles. 



The ventricle is distended during its period of rest by the flow 

 of the blood from the veins and by the auricular contraction ; and 

 its contraction seems continuous with that of the auricle, so imme- 

 diately does it succeed. The ventricular contraction is slower, and 

 probably completely empties the cavity. The ventricles contract 

 simultaneously. The shape of the ventricles is changed : as the 

 heart-muscle becomes hard and rigid in contraction, the section of 

 its base becomes circular instead of elliptical, as it is during re- 

 pose ; the ventricles shorten and twist to the right, and the form is 

 conical. As the organ relaxes, it turns back to its former position 

 and shape, that of a cone with elliptical base. This shortening of 

 the ventricles in contraction is compensated by the lengthening of 

 the great vessels at the base as they become distended by the load 

 of blood.* 



What are the functions of the valves of the heart ? 



In considering the functions of the valves of the heart one 

 must bear in mind constantly that the organ is a pump whose 

 office is to force the blood in one direction. There are four 

 principal valves two auriculo-ventricular. and two in the great 

 arteries, the aorta and the pulmonary artery. (1) As the ventricle 

 fills, the auricula-ventricular valves are floated up from the sides of 



* Some physiologists deny the shortening of the ventricles in systole. 



