236 COMPARATIVE PHYSIOLOGY. 



as the contraction of the heart and what it implies in the light 

 of any single factor ? That such narrow and exclusive views 

 should have heen propagated, even by eminent physiologists, 

 should admonish the student to receive with great caution ex- 

 planations of the working of complex organs, hased on a single 

 experiment, observation, or argument of any kind. 



The view we recommend the student to adopt in the light of 

 our present knowledge is, that the first sound is the result of 

 several causative factors, prominent among which are the sud- 

 den tension of the auriculo-ventricular valves, and the contrac- 

 tion of the cardiac muscle, not leaving out of the account the 

 possible and probable influence of the blood itself through 

 eddies or otherwise; nor would we ridicule the idea that in 

 some cases, at all events, the sound may be modified in quality 

 and intensity by the shock given to the chest wall during sys- 

 tole. 



ENDO-CARDIAC PRESSURES. 



Bearing in mind the relative extent of the pulmonary and 

 systemic portions of the circulation, we should suppose that the 

 resistance to be overcome in opening the aortic valves and lift- 

 ing the column of blood that keeps them pressed together, 

 would be much greater in the left ventricle than in the right; 

 or, in other words, that the ihtra-ventricular pressure of the 

 left side of the heart would greatly exceed that of the right, and 

 this is confirmed by actual experiment. 



• That there should be a negative pressure in, say, the left 

 ventricle, follows naturally enough from the fact that not only 

 are the contents of the ventricle expelled with great sudden- 

 ness, but that its walls remain (see Figs. 200 and 204) pressed 

 together for a considerable portion of the time occupied by the 

 whole systole; so that in relaxation it follows that there must 

 be an empty cavity to ftU, or that there must be an aspiratory 

 effect toward the ventricle; hence also one factor in the closure 

 of the semilunar valves. 



It thus appears that the heart is not only a force-pump but 

 also to some extent a suction-pump ; and, if so, the aspirating 

 effect must express itself on the great veins, lacking valves as 

 they do, at their entrance into the heart ; hence, with each 

 diastole the blood would be sucked on into the auricles, a 

 result that is intensified by the respiratory movements of the 

 thorax. 



