THE VASCULAR MECHANISM. 1 99 



amount of blood expelled from the ventricles (output of the heart) at 

 each beat, and R is the resistance against which the heart is working ; 

 R is approximately represented by the average arterial pressure. 



The output of the heart may be indirectly measured by enclosing 

 the heart in an apparatus (cardiometer) which fits closely round the 

 base of the ventricles, and is connected with some form of tambour 

 and recording lever. When the ventricles contract and expel blood 

 into the arteries, their volume diminishes, and, since the apparatus is 

 air-tight, there is a corresponding fall of the recording lever. 



One of the simplest forms of cardiometer is a glass vessel resembling 

 a large thistle funnel, the mouth of which is covered by a rubber 

 membrane with a hole in its centre. When the heart is placed in the 

 cardiometer, the border of the membrane fits closely in the auriculo- 

 ventricular groove, forming an air-tight junction. The cardiometer is 



FIG. 70. Piston recorder. (Diagrammatic. 



attached to a piston recorder, which is more sensitive than an ordinary 

 tambour, and consists of a vulcanite piston fitting closely in a cylinder, 

 the latter having an opening at its lower end by which it can be 

 connected with the cardiometer : a light counterweighted lever is 

 attached to the piston (fig. 70). The piston moves very easily, and its 

 excursions are proportional to the changes in volume of the heart. In 

 order to obtain a quantitative measurement of the output of the heart 

 the instrument is calibrated. 



Another and direct method of determining the output of the heart is 

 the heart-lung preparation (fig. 71), devised by Knowlton and Starling. 

 The common carotid artery, the descending aorta, and the inferior vena 

 cava are ligatured, and cannulse containing a solution of hirudin in normal 

 saline solution are placed in the innominate artery and the superior 

 vena cava. The blood leaving the left ventricle through the innominate 

 artery passes through a thin rubber tube A ; this can be compressed to 

 any desired extent by means of a pump C and pressure bottle D, the 

 resistance thus offered to the flow of blood through the tube replacing the 



