46 A MANUAL OF VETERINARY PHYSIOLOGY 



and the pressure against which it is propelled. The amount 

 pumped out at each systole of the ventricle is liable to great 

 variation ; at least, such are the results of experiments on the 

 dog, in which animal it has been shown that the contraction 

 volume of the left ventricle diminishes as the size of the animal 

 increases. It is obvious that the right ventricle does less work 

 than the left, for the reason that it has to pump the same volume 

 of blood against a much smaller peripheral resistance ; it has been 

 said, indeed, that the right heart does one-quarter the work of 

 the left. Colin placed the impulsive force of the right ventricle 

 of the horse at 33 kilogrammes (72*6 pounds), and of the left at 

 132 kilogrammes (290*4 pounds). 



If we take the amount of blood pumped at each stroke into 

 the aorta of the horse as about 1 kilogramme (2*25 pounds) in 

 weight, and the pressure under which it is forced upwards as 

 equivalent to a column of blood 3-048 metres (10 feet) in height, 

 then the work of the left ventricle at each stroke is equal to 

 10 kilogrammes (22*5 pounds) raised 0304 metre (1 foot) high, 

 or for twenty-four hours, allowing the work of the right heart to 

 be one-fourth that of the left, 212,275-86 kilogramme-metres 

 (1,539,000 foot pounds). This amounts to about one-thirtieth 

 of a horse-power per diem ; Munk places it at one- thirty-sixth of 

 a horse-power. If the amount of blood expelled by the left 

 ventricle at each stroke be equal to 1 kilogramme, then in a state 

 of repose the entire blood in the body of a horse passes through 

 the heart in about thirty beats, or in forty-five seconds. Munk 

 says that in the horse the entire blood passes through the heart 

 in fifty seconds, in the ox in forty seconds, and in the dog in 

 twenty seconds. Since the amount of work performed by the 

 heart is increased during exercise, the above calculations are for 

 a horse in a state of repose. 



Coronary Circulation. — The vascular system which supplies the 

 heart substance is lodged in grooves in its wall, and much dis- 

 cussion has arisen, not only as to the moment at which the 

 arteries receive their supply, but also as to the effect on the 

 coronary bloodvessels of the contraction of the surrounding 

 heart muscle. It is now generally admitted that the coronary 

 arteries receive their blood during the ventricular systole, and 

 not, as was originally thought, during the closure of the aortic 

 valves. The latter, as we have seen, do not cover the openings 

 of the coronary arteries. The question of the effect of the heart's 

 contraction on the vessels lodged in its walls is far more difficult 

 of solution. If the squeezing of the heart will assist the passage 

 of blood in one direction it must retard it in another, for the 

 veins and arteries lie side by side, and their blood is flowing in 

 opposite directions ; the effect on the thick-walled arteries is, 



