PHYSIOLOGY OF THE CIRCULATION. 105 



cavities, the mode of action of that cause neverthe- 

 less differs somewhat in the two instances. For when 

 the long arm of a syphon is once filled, the gravita- 

 ting column of liquid within it instantly acts with its 

 full power in inducing a tendency to the formation 

 of a vacuum in the portion of tube immediately 

 behind it ; whereas, the act of inspiration being a 

 gradual effort, the enlargement of the thorax occupies 

 a perceptible interval of time. And therefore, 

 though the partial vacuum formed within the latter 

 may ultimately be capable of raising a column of 

 water to the height of twenty inches, it by no means 

 follows that the derivative force acting on the con- 

 tents of the large venous trunks is, at any one moment, 

 equal to that of a gravitating column of water of this 

 height. On tne contrary, as this imperfect vacuum 

 is continually being removed by the influx of addi- 

 tional blood into the thorax, it appears tolerably evi- 

 dent that during an ordinary inspiration this deriva- 

 tive force does not at any one time exceed that of a 

 falling column of water of the same area as the large 

 veins, and three or four inches in height. And if the 

 truth of this remark be admitted, there will then be 

 a tolerably close approximation between the deriva- 

 tive force acting in the living body and that exercised 

 by the syphon in these experiments. 



As proving that the absorption and discharge of 

 the stagnant fluid did not depend upon the direct 

 action of the syphon, I may mention that the syphon 

 acted so long only as the stream continued to flow 

 through the apparatus. For when that flow ceased 

 the membranous tube was pressed against the orifice 



