THE CAUSATION OF THE HEART-BEAT 



s, is next attached to the reservoir containing the main perfusion fluid, and the 

 tube B filled with fluid and left to warm while the heart is being prepared. 



The heart having been excised and washed well in saline so as to remove 

 as much blood as possible, the cannula is tied into the aorta. The cannula is now 

 held under the perfusion tube, filled with the warm saline, and at once attached 

 in its proper position and the perfusion started. A bent pin to which a long 

 thread is tied is hooked into the apex of the heart, and the perfusion tube pulled 

 up until the heart lies quite within the warm chamber. When thus drawn up 

 the bulb w lies just below the surface of the water in the outer jacket. The 

 tube is held firmly in position by a clamp which fixes one arm of the handle F. 

 The heart cannula is provided with a side opening v, on to which a long piece 

 of fine rubber tubing is passed. This renders possible the removal of any gas 



^ 



FIG. 445. Volume curve of ventricles (cat) (lower curve). The upper curve 

 is the arterial pressure, maintained by an adjustable resistance at 

 130 mm. Hg. Between the arrows the air used for artificial respiration 

 was replaced by a mixture containing 20 per cent. CO., and 25 per cent, 

 oxygen. Note the dilatation with impaired contraction, followed by increased 

 amplitude of contractions. 



bubbles that may collect in the cannula, or the washing out of the cannula with 

 a stream of fluid if necessary. The beats of the heart are recorded by means of a 

 simple lever attached by the thread previously fixed to the heart. 



THE SIGNIFICANCE OF CARBON DIOXIDE FOR THE 



HEART-BEAT 



The blood of mammals always contains a certain amount of 

 carbon dioxide, the tension of this gas in the arterial blood of man 

 varying between 5 and 6 per cent, of an atmosphere. Henderson has 

 shown that if artificial respiration be maintained so vigorously as to 

 wash out the carbon dioxide from the blood, the heart's action is 

 modified, the rate being much quickened and the relaxation of the 

 ventricle during the diastole being incomplete, the blood pressure 

 falling in consequence of the inadequacy of the heart's action. 



It is easy, by means of the isolated lung-heart preparation described 

 on p. 1028, to demonstrate the importance of a certain tension of carbon 

 dioxide for the normal beat of the heart. The heart is placed in a 



