230 PHYSIOLOGY CHAP. 



earlier, that the pulsations in the oesophagus are considerably 

 changed and simplified after opening of the thorax. 



Martius (1888), on the other hand, while almost contemporary 

 with Fredericq, considered only the effects of meiocardia and auxo- 

 cardia in interpreting the oesophageal pulsations, and thus came 

 to a one-sided conclusion in the opposite direction. 



XL Before leaving this interesting subject of cardiac 

 mechanism, it is necessary to form some approximate conception of 

 the work that is usually performed by the heart in a unit of time. 



The work of the heart is equal in the time unit to the weight 

 of the blood it is capable of moving, multiplied by the height of 

 the pressure to which the said weight is lifted. To determine this 

 work it is necessary to form a proper appreciation on the one 

 hand of the mechanical value of each cardiac revolution, i.e. of the 

 quantity of blood that passes from veins to arteries in the said 

 unit of time ; on the other, of the height of pressure in the arteries 

 nearest to the heart. 



The weight of blood driven into the arteries at each systole 

 depends both on the degree of diastolic filling (auxocardia), and on 

 the degree of systolic evacuation (meiocardia). It is very difficult 

 to arrive at an exact determination of this weight, since it varies 

 with varying conditions. The values of 185*5 and of 180 grins, 

 cited by Volkmann and Yierordt in calculating the work of the 

 heart are certainly exaggerated. The values adopted by Fick 

 (50-75 grms.), by Tigerstedt (50 grins.), and by Zuntz (60 grms.) 

 are probably nearest to the truth. 



On the other hand, the mean pressure of the aorta may be taken 

 as approximately equal to 150 mm. Hg, i.e. in round figures to a 

 column of blood 2 metres high, and the pressure in the pulmonary 

 artery may be taken as about a third of the aortic pressure. 



Assuming that the left ventricle drives some 60 grms. of blood 

 into the aorta at each systole under a pressure of 2 in. of blood, and, 

 allowing for the velocity acquired by the same, which is a negligible 

 quantity, we obtain a yield of 120 grammetres at every revolution. 

 , Given 72 revolutions per minute, it is easy to calculate that the 

 work done by the left ventricle in 24 hours represents about 

 12,450 kilograrnmetres. If a third of this, -i.e. 4150 kilogram- 

 metres, be added as the approximate work of the right ventricle, 

 the total work of the heart in 24 hours may be reckoned as 

 16,600 kilograrnmetres. 



The friction to which the blood is submitted in its passage 

 through the closed vascular system transforms the entire work of 

 the heart into heat. Starting from the fact that 425 kilogram- 

 metres are necessary for the development of 1 calorie, the 16,600 

 kilogrammetres of the heart's daily work represent some 39 calories, 

 corresponding with the heat developed by the combustion of less 

 than 5 grms. of carbon. 



