THE CIRCULATION. 249 



PULSATIONS PER MINUTE. 

 AGE. 



MALE. 



Foetal 130 to 140 



1 year 120 to 130 



2 years 105 



3 years 100 



4 years 97 



5 years 94 



10 years 90 



10 to 15 years . . 78 



15 to 50 years 70 



80 to 90 years 80 



Pathological Action. Increasing the rate of the heart does not 

 mean an increase in work, for the output per systole is propor- 

 tionately lessened. A dilated heart must use more force that it may 

 expel the blood than one of normal capacity; hence it enlarges. 

 Dilatation and hypertrophy usually go together. 



In physiological conditions the different cavities of the heart expel 

 equal volumes of blood in the same time. Not to do so would cause 

 an unusual effort by the heart-muscle and the usual heart failure. 



The increased frequency of the heart in children is probably due 

 to want of inhibitory power in the vagus. The erect position increases 

 the heart-beat,, because the attitude makes the blood-pressure fall and 

 the heart beats more rapidly to make up for it. 



Slow pulse or bradycardia may be due either to lessened excita- 

 bility of the cardiac muscle or an exaggerated inhibition by the vagus, 

 seen in recovery from enteric fever and pneumonia. Slow pulse may 

 be due to high arterial tension, stimulating the center of the vagus. 



Emotional causes acting upon the accelerators will not raise the 

 heart-beat beyond 120 per minute. Shortening of diastole is injurious 

 to the heart, as it is the only resting time for the heart. 



W orlc of the Heart. When a force produces acceleration, or when 

 it maintains motion unchanged in opposition to resistance, it is said 

 to do work. To convey an impression of the amount of work done by 

 any machine, it is usual to express its efficiency in terms of work-units. 

 This is a comparatively easy task when attempted in the physical 

 world, but becomes extremely difficult when one attempts to express in 

 terms of work-units the force of the heart's action. The work of the 

 heart central pump, that it is is so hard to reckon in view of the ill- 

 defined data that we are able to obtain as to the resistance which it 

 overcomes, and from the fact that different portions of this human 

 machine are known to exert different degrees of force. 



