336 



OF THE CIRCULATION OF THE BLOOD. 



tids or the two crurals, the level of the mercury on the two sides remains 

 equal, indicating equality of pressure ; whilst if one be placed in a more 

 distant artery or into a Vein, the mercury constantly rises on that side, thus 

 showing a diminution of pressure or tension. If the contracted or capillary 

 portion of Marey's apparatus (Fig. 137) be enlarged, the fluid will pass 

 through it with less resistance, and the pressure in the Arterial portion will 

 diminish, and vice versa. This has actually been observed by Bernard in 

 the living subject ; for on placing the mouthpieces of his differential Manom- 

 eter into the two facial arteries of a horse, the mercury stood at the same 

 level in each arm of the instrument ; but on division of one of the cervical 

 sympathetic nerves, the mercury instantly rose on that side, indicating a 

 diminution in the tension of the vessel. Now it has been ascertained by 

 M. Brown-Sequard that section of the sympathetic causes a dilatation of the 

 Capillary vessels, and therefore facilitates the passage of the blood through 

 them ; and the counterproof is shewn by galvanizing the cut extremity of 

 the Sympathetic, which inducing contraction of the Capillaries, increases 

 the resistance to the passage of the blood from the arteries, and augments 

 the tension of their walls, as is indicated by the gradual falling of the mer- 

 cury on the galvanized side, till the pressure there may even exceed that of 

 the healthy one. 



258. The absolute pressure of the blood in the living vessels has engaged 

 the attention of many observers. Hales 1 endeavored to ascertain it by in- 

 serting long pipes into the vessels of living animals, and measuring the 

 height to which the blood rose. In a Mare, when the tube was introduced 

 into the crural artery, it rose 8 ft. 3 in. ; in two Horses, 9 ft. 8 in. and 9 ft. 

 6 in. From parallel experiments upon sheep, oxen, dogs, and other animals, 



and from the comparison of the calibre of their re- 

 spective vessels with that of the human aorta, Hales 

 concluded that the usual force of the heart 



FIG. 139. 



in man 



would sustain a column of blood 7j ft. high, the 

 weight of which would be about 4 -Ibs. 6 oz. on the 

 square inch. Poiseuille, improving on Hales's method, 

 employed a much more portable instrument in the 

 form of a doubly-bent tube (Fig. 139), containing 

 mercury in the curve, and a solution of carbonate of 

 soda in the mouthpiece, which prevented the forma- 

 tion of clots. A still further improvement was made 

 by Ludwig and Volkmanu, by placing Poiseuille's 

 instrument -in connection with a revolving drum, on 

 which tracings indicating the fluctuations of the pres- 

 sure could be taken. The instrument so modified is 

 called the " Kymographion," and is represented in 

 Fig. 140. The tracing immediately below gives a normal tracing of arte- 

 rial pressure obtained with the mercurial Kymograph from a rabbit. Poi- 

 seuille estimated the pressure in the arteries at 6.3 inches of mercury on the 

 square inch, which he assumed as the standard for all arteries, and for all 

 Mammalia. But the tracings of Ludwig and Volkmann showed that the 

 range of variation is very wide, being in the carotid of the Horse from (> to 

 13 inches, in the Dog from 7-8 inches, and in the Rabbit from 2-3 i- inches; 

 in man it is probably about equal in the larger arteries to a column of 

 mercury 8 inches in height (Ranke, 5 or 6 Faivre). Bernard has invented 

 a lighter instrument, termed the " Cardiometer," the indications given by 

 which are somewhat higher than those obtained by Poiseuille's apparatus. 



Poiseuille's HBeniadynamoin- 

 eter: A, mouthpiece. 



1 Haemastaties, vol. i, pp. 1-GO. 



