vni BLOOD-STEEAM : MOVEMENT IN VESSELS 253 



the volume of blood driven by the heart into the arteries at each 

 systole. Even if blood pressure in the horse differs little from 

 that of the rabbit, yet the horse's heart does far more work, 

 because it throws a far greater quantity of blood into circulation. 



The mean blood pressure in man cannot differ much from that 

 of the larger mammals. We may think of it as varying approxi- 

 mately from 130 to 150 mm. Hg, values which corae very near 

 those which Faivre (1850) determined directly upon the femoral 

 and brachial arteries in amputations. 



The values obtained by Marey and Mosso by their sphygmo- 

 rnanornetric methods reach no such high figures (80 to 90 mm. 

 Hg) ; yet it must be remembered that they only represent the 

 average total pressure of all the arteries, large and small, of the 

 forearm, or of the fingers of the hand. 



The mean arterial pressures, as determined by Eiva-Eocci 

 with his sphygmornanometer, oscillated in healthy individuals 

 between 125 and 135 mm. Hg. They corresponded to the 

 greatest total pressure obtained in the brachial artery of man 

 when a simple cannula is introduced in a central direction, and 

 connected with Ludwig's kymograph. As we have said, these 

 values represent approximately that of the lateral pressure that 

 prevails in the aorta. 



The blood pressure sinks very little between the aorta and the 

 larger branches of the arteries, in which it is possible to introduce 

 a cannula connected with a manometer, because the resistance due 

 to the friction surfaces increases very little. Volkmann gives the 

 following data for dog and calf, which correspond essentially with 

 those obtained by Bernard, Marey, and others : 



Carotid of dog = 172 mm. Hg. '" Carotid of calf =116 mm. Hg. '' 

 Femoral =165 Femoral = 116 



Metatarsal =155 Metatarsal = 88 



On the other hand, pressure falls rapidly in the small branches 

 of the arteries, where there are many ramifications, and in the 

 capillaries, where the friction .surfaces are greatest. The values 

 obtained by v. Kries with Ludwig's method of counter-pressure 

 can only be taken as approximate. Generally speaking, we may 

 assume that pressure in the capillaries does not exceed -|-f- of 

 the aortic pressure (about 20-38 mm. Hg). 



In the trunks of the veins nearest the heart, as in the in- 

 nominate, subclavian, and jugulars, there is on an average a 

 negative pressure, which according to Jacobsen may attain a 

 value of -O'l mm. Hg, and is due to the aspiration exerted by the 

 lungs in consequence of the elastic tension into which they are 

 thrown during the expiratory phase. The pressure becomes 

 positive in the veins farthest from the heart and thorax. Accord- 

 ing to Jacobsen it amounts to 0'3 mm. in the external facial vein 



