58 CIRCULATION OF THE BLOOD 



the fineness of the column in the tube, some of the inconveniences due 

 to the weight of mercury in the hemodynamometer are avoided, and there 

 is, also, less friction. This instrument is appropriately called the cardi- 

 ometer, as it indicates accurately, by the extreme elevation of the mercury, 

 the, force of^iiie -heart ; but it is not so useful in measuring the mean 

 arterial pressure, for in the abrupt descent of the mercury during the 

 diastole of the heart, the impetus causes the level to fall below the real 

 standard of the constant pressure. Marey corrected this difficulty in the 

 " compensating " instrument, which is constructed on the following prin- 

 ciple : Instead of a simple glass tube which communicates with the mer- 

 cury in the bottle, as in Magendie's cardiometer, there are two tubes ; 

 one is like the tube already described and represents oscillations pro- 

 duced by the heart, while the other is larger, and has, at the lower part, 

 a constriction of its calibre, which is here reduced to capillary fineness. 

 The latter tube is designed to give the mean arterial pressure ; the con- 

 stricted portion offering such an obstacle to the rise of the mercury that 

 the intermittent action of the heart is not felt, the mercury rising slowly 

 to a certain level, which is constant and varies only with the constant 

 pressure in the vessels. 



The instruments in use in physiological laboratories at the pres- 

 ent day are modifications of the " kymograph " devised by Ludwig. 

 Ludwig's instrument consists of a U-tube containing mercury, one 

 arm of which is connected with an artery. The mercury in the distal 

 arm carries a float to which is attached a wire terminating in a writing- 

 point by means of which the oscillations may be recorded in the usual 

 way. The principle, however, is the same as that of the instrument 

 constructed by Poiseuille. 



Pressure in Different Arteries. The experiments of Hales, Poi- 

 seuille, Bernard and others seem to show that the constant arterial 

 pressure does not vary much in arteries of different sizes. These 

 physiologists experimented 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 nearly equal, in all parts 

 of the arterial system. The experiments of Volkmann, however, have 

 shown that this conclusion is not correct. With the registering ap- 

 paratus of Ludwig, he took the pressure in the carotid and the meta- 

 tarsal 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 millimeters) in the carotid, and 6.6 inches 

 (165 millimeters) in the metatarsal. In an experiment on a calf, the 

 pressure was 4.64 inches (116 millimeters) in the carotid, and 3.56 

 inches (89 millimeters) in the metatarsal; and in a rabbit, 3.64 inches 



