G. NATURAL HISTORY AND ZOOLOGY. 285 



the gases boiled off from water brought up from great depths 

 in the Mediterranean contained only about five per cent, of 

 oxygen and thirty-five per cent, of nitrogen, the remaining 

 sixty per cent, being carbonic acid ; whereas in gases obtain- 

 ed from the deep waters of the Atlantic the average percent- 

 age of oxygen was about twenty, while that of carbonic acid 

 was between thirty and forty, this large proportion of car- 

 bonic acid not appearing prejudicial to the life of marine in- 

 vertebrata so long as oxygen was present in sufficient propor- 

 tion. The physical cause of this deficiency of oxygen and 

 excess of carbonic acid is found in the absence of any bottom 

 circulation, the whole interior of the sea being in an abso- 

 lutely stagnant condition. The circumstances that produce 

 circulation in the ocean are not present here, there being no 

 possibility of an increase of the density of the surface stratum 

 by the reduction of temperature, involving its sinking to the 

 bottom, to be replaced by the bottom water coming to the 

 top. On this account the bottom water is never disturbed, 

 and the organic matter contained in the sediment accumula- 

 ted there consumes its oxygen so much more rapidly than it 

 can be supplied from above, and diffused through the vast 

 column of superincumbent water, that nearly the whole of 

 it is converted into carbonic acid, scarcely any of the oxy- 

 gen being left for the support of animal life. The existence 

 of a reef across the Strait of Gibraltar also effectually pre- 

 vents any circulation from the Atlantic. 18 A, August 29, 

 1873,604. 



THE MECHANICAL PERFECTION OF THE HUMAN HEART. 



Mr. Garrod, in an interesting lecture at the Royal Institu- 

 tion of London, on the heart and the sphygmograph, states 

 that the heart is to be viewed as a pump, constructed on the 

 same principle as an engine built as indicated by himself, and 

 so regulated by means of an elastic bag that the velocity of 

 the working machinery does not vary with the work to be 

 done. A large number of measurements has enabled him to 

 show that the relative lengths of the systolic and diastolic 

 portions of the pulse trace do not vary for any given pulse 

 rate, but that the blood pressure in the arteries is quite inde- 

 pendent of the pulse rate ; whence it follows that the force of 

 the cardiac muscular contraction varies directly as the blood 



