CHAP. XXVIII.] RATE OF THE CIRCULATION. 381 



in them. It is important that this fact should be kept in view by 

 the practitioner in the treatment of varicose veins, and of ana- 

 sarcous states of the limbs. 



We have seen that the blood moves in the arterie s with con- 

 siderable velocity, and likewise with great, although much di- 

 minished, rapidity in the capillaries ; its rate of motion increasing 

 again in the veins, especially in those nearest the heart. It may be 

 inferred from these facts, that any given particle may complete the 

 round of the circulation in an exceedingly brief period. It is an 

 important problem especially with reference to the time in which 

 poisonous substances introduced into the blood may produce their 

 effect to determine in what space of time a substance introduced 

 into one part of the circulation may reach the most distant part ; or 

 how soon, for instance, a substance inserted into the right jugular 

 vein may, after traversing the right heart, the pulmonary circula- 

 tion, the left heart, the systemic arteries, return to the systemic 

 veins, and be found in the left jugular vein. 



In the present state of our knowledge no exact solution of this 

 problem can be given. But a very close approximation to the 

 truth may be obtained, first by calculation, secondly by experi- 

 ment. By calculation we can determine in what space of time 

 the whole blood of the body may circulate through the heart. The 

 data for this problem are, the weight of the whole quantity of 

 blood in the body, the quantity of blood expelled at each systole 

 from the left ventricle, and the number of systolic actions in a 

 minute, or, more exactly, the duration of a pulse. It is plain that 

 if the left ventricle contract seventy times in a minute, and at each 

 contraction expel five ounces of blood, according to Valentin, or 

 six ounces (-j-J^th part of the weight of the body), according to 

 Volkmann, a quantity of blood equal to that of the whole body 

 will in that space of time pass through the heart. It may, then, 

 be assumed from calculation, that the circulation may be completed 

 in a period of time, which, in round numbers, may be expressed as 

 one minute.* 



* Volkmann's formula is t z - where t is the required mean time of the 



9 



completion of the circulation, # is the whole quantity of the bloody the quantity 

 expelled at each systole, and z the mean duration of a pulse. "Whence, taking 

 the mean quantity of x at 30 Ibs., and of y at 6'2 ounces, the duration of a pulse 



being 0-85 of a second, we get t = 0'85 ^y = 67*5 seconds. And as, ac- 



188 



cording to Valentin, the whole quantity of blood is equal to about l-5th the 



