148 



ANIMAL MECHANICS. 



a vertical column of blood, which was noticed to rise and fall. 

 Subsequent observers have shown that in the anterior extre- 

 mity, the blood pressure depends on the respiration, being 

 greatest during expiration, and least during inspiration ; and 

 that this inequality is less sensible in the arteries distant from 

 the heart. In order to compare the observations, it is desirable 

 to reduce Dr. Hales' column of blood to millimeters of mer- 

 cury. The weight of a cubic inch of blood is 267.7 grs., 

 while that of a cubic inch of water is 252.5 grs. The specific 

 gravity of mercury is 13.596; and 9.14 feet (Dr. Hales' result 

 for the horse) are equal to 2785 millimeters. Hence we find 



Maximum mean pressure of blood in the horse, according to Hales, 



2785x267.7 



= 7 = 217.17 mm. of mercury. 



13.596 x 252.5 ' 



Poiseuille* directed his attention especially to the deter- 

 mination of the mean pressure of the blood, independent of 

 expiration or inspiration ; but has left in his memoir (p. 295), 

 the data for calculating, in the case of the horse, the maximum 

 pressure corresponding to that observed by Dr. Hales. From 

 the experiment there recorded, on the carotid artery of a 

 horse, I have calculated the maximum mean pressure of the 

 blood during expiration, and find 



Maximum mean pressure in the carotid artery of the horse, according to 

 Poiseuille, 



= 229.75 mm. of mercury. 



The minimum mean pressure, during inspiration, in the 

 same artery, may be calculated from the data given, at 63.61 

 mm. of mercury. The mean of all the maxima and minima 

 being 146.68 mm. of mercury. The maximum pressure found 

 by Poiseuille is a remarkable confirmation of the accuracy of 

 the experiments made by Dr. Hales. 



* "Journal de Physiologie par Magendie," torn, viii., p. 272. 



