BY DR. BURDON-SANDERSON. 271 



pumped through it continuously, the observer has it in his 

 power to modify the arterial pressure (by altering the resist- 

 ance) without modifying the venous pressure, and vice versa, 

 and so to reproduce conditions which actually exist and exer- 

 cise a most important influence in the living body. 



It is obvious that if the pressure on the venous side of the 

 heart is ?u7, no progressive movement will occur, whatever may 

 be the resistance in the arteries; and, on the other hand, that 

 if the pressures .on the two sides of the heart are equal, there 

 must also be no movement, for, the auriculo-ventricular valve 

 remaining open, the heart would act as in the previous experi- 

 ment, receiving back again in diastole whatever liquid it dis- 

 charged during systole. Between these two extremes, that of 

 equality of venous and arterial pressures and that of total 

 want of pressure in the auricles, a mean relation exists which 

 is most advantageous to efficient action, and cannot be de- 

 parted from in either direction without impairment of effect. 

 The existence of this ratio of greatest efficiency has been lately 

 demonstrated experimentally by Blasius; 1 and it has been found, 

 first, that for every value of arterial resistance, it is possible by 

 successive trials to ascertain what venous pressure enables the 

 heart to contract with the greatest effect ; and, secondly, that 

 for every heart there is a certain value of arterial resistance 

 which is most advantageous. The mean result of numerous 

 observations is, that the frog's heart (ranaesculenta) does most 

 work when it is opposed by an arterial pressure of about 35 

 millimetres of mercury. If the resistance is greater than this, 

 the heart becomes over-distended, and its valves incompetent. 



66. Application of the preceding Methods to the 

 Investigation of the Problem of the Mechanical 

 Work done by the Heart in a given Time. In the 

 preceding paragraph, the expressions, mechanical " effect" of 

 the heart's contractions, and " work" done by the heart, have 

 been used without explanation. Before proceeding further, it 

 is necessary to define them. The work done b}' the heart in 

 any given time is equal to the product of the aortic pressure 

 and the quantity of blood which passes through the aortic 

 orifice in the same time. To illustrate this, it is necessar}' to 

 revert to the experiment described in 46, in which the circu- 

 lation is maintained artificially in the frog by substituting for 

 the heart a column of serum of sufficient height. In this case, 

 so long as the height of the column remains unaltered, the 

 work done in canning on the circulation truly represents that 

 of the heart. If it is allowed to diminish, the rate of flow 

 diminishes with it. To maintain constancy in the circulation, 



1 Am. Frosch-Herzen angestellte Versuche uber die Herz-Arbeit, etc. 

 Pick's Arbeiten, Wurzburg, 1872, p. 1. 



