PEESSURE OF BLOOD IN THE ARTERIES. 



73 



is constructed on the following principle: Instead of a simple glass tube 

 which communicates with the mercury in the bottle, as in Magendie's 

 cardiometer, there are two tubes, 

 one of which is like the one 

 already described and represents 

 oscillations produced by the 

 heart, while the other is larger, 

 and has, at the lower part, a 

 constriction of its caliber, which 

 is here reduced to capillary fine- 

 ness. The latter tube is de- 

 signed to give the mean arterial 

 pressure ; the constricted portion 

 offering such an obstacle to the 

 rise of the mercury that the in- 

 termittent action of the heart is 

 not felt, the mercury rising slow- 

 ly to a certain level, which is 

 constant and varies only with 

 the constant pressure in the ves- 

 sels. 



Physiologists have only an 

 approximative idea of the arte- 

 rial pressure in the human sub- 

 ject, derived from experiments 

 on the inferior animals. It has 

 already been stated to be equal 

 to about six feet (183 centime- 

 tres) of water or six inches (150 

 mm.) of mercury. 



Pressur-e in Different Arteries. The experiments of Hales, Poiseuille, 

 Bernard and others, seem to show that the constant arterial pressure does 

 not vary much in arteries of different sizes. These physiologists experi- 

 mented particularly on the carotid and crural, and found the pressure in 

 these two vessels about the same. From their experiments they concluded 

 that the force is equal in all parts of the arterial system. The experiments 

 of Volkmann, however, have shown that this conclusion is riot correct. With 

 the registering apparatus of Ludwig, he took the pressure in the carotid 

 and the metatarsal arteries and always found a considerable difference in 

 favor of the former. In an experiment on a dog, he found the pressure 

 equal to about seven inches (172 mm.) in the carotid, and 6*6 inches (165 

 mm.) in the metatarsal. In an experiment on a calf, the pressure was 4*64 

 inches (116 mm.) in the carotid, and 3*56 inches (89 mm.) in the meta- 

 tarsal ; and in a rabbit, 3'64 inches (91 mm.) in the carotid, and 3'44 inches 

 (86 mm.) in the crural. These experiments show that the pressure is not ab- 

 solutely the same in all parts of the arterial system, that it is greatest in the 



FIG. 29. Compensating instrument of Marey. 



