204" CIRCULATION OF THE BLOOD 



vantage obviated. With a breadth of about 15 cm. v. Eecklinghausen observed 

 in eight trials a variation of only 3 mm. Hg. in the blood pressure determined 

 in the brachial and the femoral arteries at the same time. (In animal experi- 

 ments the pressure in these two is in general found to be the same.) Such a 

 method therefore gives very satisfactory results, as it is also much more easily 

 manipulated than other methods thus far devised for this purpose. 



[With Erlanger's apparatus (Fig. 76) it is possible also to determine the 

 maximum systolic and minimum diastolic pressures. To determine the former 

 the arm is compressed until no pulse can be felt in the radial artery. Even at 

 this time the tambour of the instrument shows vibrations due to pulsations in 

 the central stump of the artery ; but if the air pressure on the arm be now 

 lowered gradually by means of an escape valve (V in the Figure) these vibrations 

 will suddenly become larger. The pressure which the manometer shows when 

 this takes place is the pressure which the pulse wave can just overcome and is, 

 therefore, the maximum systolic pressure. If the pressure be lowered still further 

 the vibrations shown by the lever of the tambour will become still larger until 

 a point is reached at which they begin to decrease. The pressure at which the 

 arterial wall makes the widest fluctuations, and the lever therefore its largest 

 vibrations, is the minimum diastolic pressure. ED.] 



C. HEIGHT OF THE BLOOD PRESSURE 



Since the normal blood pressure in the aorta? of different mammals shows 

 but relatively slight differences, we can form an approximate picture of the 

 blood pressure in man from the numerous determinations made directly upon 

 different species of animals. The normal pressure in the dog is 130-180 

 mm. Hg., in the rabbit 80-120, in the horse 150-200. We may say, there- 

 fore, that the mean normal blood pressure in man varies between 100 and 200 

 mm. Hg., and if we wish to use a single figure we may assume that 150 

 mm. is the most probable value. 



We are not, however, to regard the blood pressure as constant; on the 

 contrary very considerable variations make their appearance on slight provo- 

 cation. We have, therefore, to study the factors upon which the blood pressure 

 depends. 



These are essentially three: the energy of the heart, the resistance in the 

 arteries, and the total volume of the blood. 



1. The Energy of the Heart. The quantity of blood which the heart 

 expels in a given time may be taken as a measure of its energy. If, other 

 things being equal, the quantity expelled in a given time decreases, the blood 

 pressure falls as is the case for example upon stimulation of the vagus (Figs. 

 7779. If, on the other hand, the decrease in pulse rate is only slight, and 

 is compensated by a larger pulse volume (cf. page 189), the mean blood 

 pressure falls only a little or not at all (Fig. 77). 



The quantity of blood expelled from the heart may decrease also without 

 a fall in the frequency of heart beats (cf. page 188), in which case of course 

 the blood pressure falls. 



On the other hand the energy of the heart may increase without any 

 change in frequency of the pulse (cf. page 192), and a rise in pressure results. 



When the heart is accelerated by division of both vagi, or by stimulation 

 of the accelerator nerves, it may or may not expel more blood in a given 



