490 OF THE CIRCULATION OP THE BLOOD. 



turned as to shut off all communication with the glass tube. As soon as the 

 instrument has been properly fixed in the artery, the blood is allowed to flow 

 into the glass tube. It may be now seen to traverse the glass tube with a 

 velocity very nearly the same as it has in the artery, and in doing so, it 

 pushes the water before it into the peripheral bloodvessels, with (according to 

 Volkmann) only a very slight admixture between the two fluids. 



By trials made with his haemodronieter, Volkmann found, in the case of 

 seven dogs, that the blood flowed in their carotids with a velocity ranging be- 

 tween 205 and 357 millimetres in a second; in that of horses, 306 to 234; in 

 the metatarsal artery of the horse, 56, and in the maxillary artery of the same 

 animal, 99; in the carotid of a calf, 431. The average velocity in the carotids of 

 mammals is stated by Volkmann to be 300 1 millimetres in a second. 3 ED.] It 

 appears from the observations of the Profrs. Weber already referred to ( 513), 

 that the velocity undergoes a marked increase in branches of arteries whose 

 diameter has undergone diminution by the contraction of their walls, the ac- 

 celeration being proportionate to the narrowing of the tube, as might d priori 

 be expected; a gradual retardation took place with the return of the artery to 

 the original diameter ; and when, as sometimes happened, the vessel dilated to 

 more than its former dimensions, a positive diminution in the rate of move- 

 ment of the blood was observable. 



519. The lateral pressure of the blood against the walls of the arteries was 

 aflirmed by Poisseuille to be equal throughout the whole arterial system; but the 

 more accurate experiments of Volkmann (made with Lud wig's " kymographion," 

 which is a far more trustworthy instrument than the " heemadynamometer" of 

 Poisseuille) have shown that this is far from being correct. The pressure of 

 the blood, he remarks, is no constant magnitude, but is incessantly changing 

 according to the stroke of the heart, the movements of respiration, and the mus- 

 cular actions of the body generally. A gradual diminution of its amount, how- 

 ever, may be nearly constantly traced from the commencement of the arterial 

 to the termination of the venous system; and this is to be partly accounted for 

 by the increase in the caliber of the vascular system, which takes place as we 

 pass from the arterial trunks to their ramifications ( 512), and still more from 

 the arterial to the venous system ( 530); and partly by the diminution of re- 

 sistance (which is the essential cause of the lateral pressure) as the blood moves 

 onwards towards its point of discharge ( 516). The following table presents 

 the results of Volkmann's observations (Op. cit., p. 173) upon the relative lateral 

 pressure at four points of the circulation in different animals, namely (i.) the 

 carotid near its origin, (n.) a peripheral branch of the carotid or some other 

 artery, (in.) a peripheral rootlet of a vein, and (iv.) the jugular vein. 



i. n. in. iv. 



Goat ..... 135 126 41 18 



Horse 'A ...... -j; 122 97 44 21.5 



Calf 165.5 146 27.5 9 



A blood-pressure equivalent to a column of mercury 160 millim. 6.3 (inches) 

 in height was assumed by Poisseuille as the standard for all arteries and for all 

 Mammalia, and therefore (by inference) for Man. It has been shown by Lud- 

 wig and Volkmann, however, that the range of variation is very wide, being in 

 the carotid of the horse from 150 to 321, and being not less in other animals. 

 Hence it is obvious that no precise specification can be laid down upon this point. 



1 Tolerably close approximations to the value of these measurements in English inches, 

 may be obtained by dividing each number by 25. 



2 Todd and Bowman's Physiological Anatomy, Am. Ed. 



