366 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



When this tube is fitted to the conical pipes, then the bent glass 

 tube, previously filled with water, must be fixed to it by means of 

 the short tubes (A, h' , C), the stopcock being so 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 now be 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 blood- 

 vessels, with (according to Yolkmann) only a very slight admixture 

 between the two fluids. 



By trials made with his hsemodromometer, Volkmann found, in 

 the case of seven dogs, that the blood flowed in their carotids 

 with a velocity ranging between 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"* millimetres 

 in a second. 



It results, likewise, from these observations, that the velocity of 

 the blood in the large arteries, and also in the large veins, is con- 

 siderably greater than in the capillaries ; that the velocity in 

 arteries is not uniform, but is suddenly increased at each systole of 

 the heart ; and that the blood moves most quickly in the arteries 

 nearest the heart. It appears, also, that the blood's velocity is 

 materially lessened by loss of blood, and that increased rate of 

 pulse, which always follows considerable losses of blood, is no indi- 

 cation of a more rapid blood-current, but, on the contrary, often 

 accompanies a retardation of it. The velocity of the blood-current 

 is influenced not so much by the rate of action of the heart, as 

 by the intensity of its systole, and the quantity of blood which it 

 expels at each contraction. 



Much was formerly said respecting the disposition of. the arterial 

 tree, being such that the combined areas of the branches of an 

 artery exceeded that of the trunk ; and that with each succeeding 

 series of subdivisions, the blood flows into an increased area. 

 Haller (tom.i. p. 77) ascribes the first observation of this kind to 

 an Englishman named Cole.f It is repeated by Keill, Hales, and 

 many others, among them John Hunter and Sir C. Bell. The general 

 effect of such an arrangement would obviously be to diminish the 



* Tolerably close approximations to the value of these measurements in 

 English inches may be obtained by dividing each number by '25. 

 t De Secretione aniniali. Oxon. 1674. 



