ACTION OF THE HEART. 127 



ventricles, results from tlie walls of the left ventricle being 

 about twice as thick as those of the right. And the dif- 

 ference is adapted to the greater degree of resistance which 

 the left ventricle has to overcome, compared with that to 

 be overcome by the right: the former having to propel 

 blood through every part of the body, the latter only 

 through the lungs. 



The force exercised by the auricles in their contraction 

 has not been determined. Neither is it known with what 

 amount of force either the auricles or the ventricles dilate ; 

 but there is no evidence for the opinion, that in their dila- 

 tation they can materially assist the circulation by any such 

 action as that of a sucking-pump, or a caoutchouc bag, in 

 drawing blood into their cavities. That the force which 

 the ventricles exercise in dilatation is very slight, has been 

 proved by Oesterreicher. He removed the heart of a frog 

 from the body, and laid upon it a substance sufficiently 

 heavy to press it flat, and yet so small as not to conceal the 

 heart from view ; he then observed that during the con- 

 traction of the heart, the weight was raised ; but that 

 during its dilatation, the heart remained flat. And the 

 same was shown by Dr. Clendinning, who, applying the 

 points of a pair of spring callipers to the heart of a live 

 ass, found that their points were separated as often as the 

 heart swelled up in the contraction of the ventricles, but 

 approached each other by the force of the spring when the 

 ventricles dilated. Seeing how slight the force exerted in 

 the dilatation of the ventricles is, it has been supposed 

 that they are only dilated by the pressure of the blood 

 impelled from the auricles ; but that both ventricles and 

 auricles dilate spontaneously is proved by their continuing 

 their successive contractions and dilatations when the heart 

 is removed, or even when they are separated from one 

 another, and when therefore no such force as the pressure 

 of blood can be exercised to dilate them. By such spon- 

 taneous dilatation they at least offer no resistance to the 



