146 THE BLOOD AND ITS CIRCULATION. 



pressure of the arterial system: 1, What we have called gen- 

 eral pressure, minimum pressure ; 2, The oscillations which 

 this pressure undergoes at every fresh projection of blood 

 from the ventricle. By the appreciation of this new element, 

 these rhythmic maxima, Poiseuille's idea is justified up to a 

 certain point : the pressure varies in different animals owing 

 to a variety of causes, among which size holds no unimport- 

 ant place (Cl. Bernard). 



Velocity. The velocity and the pressure of the blood at 

 any given point do not always exactly correspond : we have 

 seen that by stopping the flow of blood in a vein we increase 

 the pressure. The pressure at any given point depends on 

 the distance of this point from the extremities of the double 

 circulatory cone ; while the velocity, on the contrary, depends 

 on the form and width of that part of the circulatory cones in 

 which the point is situated. In other words, and this is easily 

 understood, the rapidity of the movement of the blood is in 

 proportion to the space contained in that part of the tube under 

 consideration. It must be remembered that we always speak 

 of the united tubes under the appellation of the double cone. 

 Thus, where the circulatory system is very large, as at the 

 base of the cones (the region of the capillaries), the blood cir- 

 culates slowly ; exactly as the current of a river slackens 

 greatly as the river widens, into a lake, for instance: thus 

 the capillaries form the lake of the blood-torrent. The max- 

 imum of the velocity is, however, attained in the narrow 

 orifices through which the blood flows, that is, towards the 

 summit of the cones in the aorta and in the vena cava. 



These conclusions have been verified by direct experiment. 

 The speed in the capillaries has been measured by microscopic 

 examination of the small vessels of the frog, for instance ; or 

 by examining with the ophthalmoscope the capillaries in the 

 retina of man ; wherein the blood globules can be distinctly 

 traced, and the time necessary for them to traverse a given 

 distance calculated ; it has thus been decided that the speed 

 in the capillaries is only from one-half to one millimetre a 

 second. This is trifling, compared with what we shall find 

 in the larger vessels, but we mut bear in mind, not only that 

 the capillary system, taken altogether, forms the lake of the 

 blood-torrent, but also that this lake is subdivided into a mass 

 of fine net-work, friction against which deprives the liquid 

 of much of its impulsive force. The influence of this tric- 

 tion, of this adherence to the walls of the capillaries, is fully 

 shown by the researches of Poiseuille on the flow of the 



