206 BIOLOGY. [Book ii. 



tumultuous acceleration of the cardiacal beatings, either primi- 

 tively, or consecutively to a retardment. "We can also dii-ectly 

 excite precipitate beatings by touching the internal wall of a 

 ventricle with a foreign body, for instance, a thermometer intro- 

 duced into the heart. (01. Bernard.) 



Apart from all these causes of perturbation, the heart exercises 

 on the blood contained in its cavities a constant and very uni- 

 form pression. In the mammifers this pression is equivalent to 

 scarcely more than a force capable of raising a column of mer- 

 cury of 150 millimetres. Contrarily to all . prevision, this pres- 

 sion is strikingly inferior to that which the blood undergoes in 

 the arteries ; and yet the blood is rapidly driven from the heart 

 into the arterial canals. M. SchiS has given an ingenious ex- 

 planation of this physiological paradox, by demonstrating expe- 

 rimentally, that the predominance of the cardiacal impulsion is 

 due to the combined action of the shock, and of the elasticity of 

 the walls of the heart.' 



We can now form a sufficiently clear idea of the functional 

 activity of the heart. The cardiacal cavities contract in couples, 

 the auricles first of all, and together, then the ventricles. 



In the simple heart of fishes the heart is never traversed except 

 by venous blood, which flows first of all into the auricle, is 

 driven by it into the ventricle, and thence by a ventricular con- 

 traction into the respiratory organs, the branchiae. In the rep- 

 tiles, in which there are two auricles and a single ventricle, the 

 arterialised blood returning from the respiratory surfaces, and 

 the venous blood charged with carbonic acid coming from the 

 tissues, blend in this ventricle common to them both. 



In the birds and the mammifers the venous blood penetrates 

 into the right auricle, passes thence into the right ventricle, which 

 drives it into the lungs. There it is oxygenised, becomes ver- 

 milion, arterial, returns to the heart ; but this time, into the 

 left heart. It penetrates into the auricle of that side, which 

 sends it forth once more into the corresponding ventricle ; whence 

 ' M. Schiff (unpublished work). 



