556 CIRCULATION OF BLOOD AND LYMPH. 



muscular work. In muscular activity there is a necessity for a 

 much larger output from the heart to supply blood and oxygen 

 to the working muscles. We know, in fact, that under these condi- 

 tions the output from the heart per minute is greatly increased ; in 

 severe exercise the volume discharged may be tenfold that given 

 out in the resting state. This large increase in work is effected in 

 part as a result of the mechanical principle described, namely, the 

 stronger contraction given by the dilated heart, and in part by an 

 increase in the heart rate brought about by reflex stimulation 

 through the cardiac nerves. The mechanical effect, that is, the 

 greater dilatation of the heart during diastole, follows directly 

 from the increased venous flow into the heart. The contractions 

 of the muscles and the increased respiratory movements combine 

 to force a larger quantity of blood into the right side of the heart 

 and under a greater pressure. As a result the ventricle is more 

 distended during diastole and responds with a greater contraction, 

 so that at each beat an increased volume of blood is discharged 

 into the artery. At the same time the rate of the heart beat is 

 increased by the nervous reflex referred to, and the combination 

 of these two effects causes an increase in the output per minute 

 which is proportional to the amount of work done. The output 

 of the heart per minute may be designated as the minute volume, 

 and it is evident that if we could determine this volume in human 

 beings during rest and work we should have a satisfactory method 

 of expressing the state of physiological efficiency of the heart. 

 While methods have been devised for estimating the minute 

 volume in man, they are not convenient nor satisfactory, and in 

 attempting to determine cardiac efficiency the physician is forced 

 to use more indirect methods, such as the effect of exercise upon 

 the heart rate and the blood-pressure. If we disregard valvular 

 defects, a poor or inefficient heart is one in which the musculature 

 is injured or is defective to some extent so that it does not respond 

 to distention by a stronger contraction or at least not to the extent 

 exhibited by the normal heart. In the case of an inefficient heart 

 the muscular exercise forces more venous blood into the heart, 

 but the latter, not responding normally, the output is not increased 

 and arterial pressure does not rise. The heart rate may increase 

 or, indeed, may be greater than normal, since this reflex accelera- 

 tion in rate is initiated normally by the rise of venous pressure in 

 the heart, and if the heart is not emptying itself normally the in- 

 crease in venous pressure will be more marked and there will be 

 a greater reflex effect upon the rate. If a patient is made to do a 

 certain stint of work, for example, to step on and off a chair a 

 definite number of times, his blood-pressure and pulse rate will be 

 affected. If the reaction given to this amount of exercise by an 



