664 SPECIAL PHYSIOLOGY. 



lowered at each diastole : the highest point indicates the full power of 

 the heart, overcoming the resistance of the column of blood, and dis- 

 tending the arterial walls ; whilst the lowest point shows that force, 

 reacting through the resilience of the arteries only. The mean height 

 between the two levels, is usually recorded as the average blood pres- 

 sure. Hales had already noticed, in his apparatus, a descent of 1 inch 

 in the blood column, between each pulsation. To determine the exact 

 force in pounds weight, the difference between the sectional area of 

 the artery experimented upon, and that of the tube containing the 

 mercury, must be noted, and the weight of a mercurial column of the 

 indicated height, but of the same area as the artery, must be deter- 

 mined by calculation. Should any blood descend into the tube, its 

 weight must be reckoned, though it is only ^th the weight of mercury. 



By means of a simple hsemadynamometer, Poiseuille found that the 

 blood pressure varied little in different sized arteries, and in different 

 sized animals ; and he concluded that 6.3 inches of mercury was, in 

 all cases, the average equivalent of pressure. This general result cor- 

 responds nearly with that calculated by Hales for Man ; thus mercury 

 being 13.6 times heavier than water, 6.3 inches of the former would be 

 equal to 85.68 inches of the latter, Hales's estimate giving 90 inches 

 of blood, which are equal to 95 of water. Again, 6.3 inches of mer- 

 cury on the square inch would be equal to 3 Ib. 2 oz. pressure, Hales's 

 estimate being equal to 3 Ib. 7 oz. 



The force of the left ventricle itself can only be estimated from 

 that observed in the arteries nearest to the heart; taking the blood 

 pressure in the aorta at 6.3 inches of mercury, then the force of the 

 left ventricle is found by multiplying that number by the square inches 

 contained on the internal surface of that cavity. 



The uniformity of pressure believed by Poiseuille to exist in arteries, 

 both near to and distant from the heart, which was thought to equalize 

 the force of the circulation in every part, and so to render congestion 

 or deficiency of blood ordinarily impossible, does not appear to pre- 

 vail. In a system of rigid tubes, the pressure would be uniform, unless 

 these were of very great length, and then only from friction. In 

 curved and resilient tubes, however, branching into vessels of rather 

 larger area than the trunks, some loss of force must be sustained. 

 Neither is it true, as Poiseuille supposed, that, in a series of animals 

 of different size, the blood pressure in the arteries is nearly uniform, 

 because, as he alleged, it is regulated by a relation between the force 

 of the ventricle, and the size of the aortic orifice. 



An adaptation of the hgemadynamometer, named the kymograpliion, 

 which yields very accurate results, has enabled more recent experi- 

 menters to correct the observations and conclusions of Poiseuille. 

 Upon the surface of the mercury in the longer leg of the ordinary 

 instrument, there rests a float, which is made to carry a vertical rod ; 

 on the upper end of this is fixed a horizontal pencil, having its point 

 resting on a drum capable of revolving upon a vertical axis. When 

 the instrument is in use, the drum is made to turn at a given rate, by 

 clockwork, and the pencil, moved by the mercury, describes a waved 

 line corresponding with the variations in the blood pressure. In this 



