150 THE BLOOD AND ITS CIRCULATION. 



does not ensue quite so soon, because the blood takes more 

 time to return by the saphenous than by the jugular veins. 

 The standard experiment consists in injecting yellow cyanide 

 into the central end of the jugular vein, and collecting the 

 blood which flows out at the peripheral end. We find that, 

 after the lapse of from eight to fifteen seconds, the poison 

 appears at this end, the blood beginning to show the charac- 

 teristic reaction of prussian blue (with salts of iron). 



Special Arrangement of the Circulatory System in some 

 Organs. Such are the general conditions of the circulation, 

 of its pressure and its velocity at different points. But the 

 system of cones which we have been considering is not 

 always everywhere so simple, and we find, in different parts 

 of the circulatory apparatus, arrangements and conditions 

 which are purely physical and mechanical, and which modify 

 the rapidity of the course of the blood. Such are the great 

 number of tubes, the clusters of capillaries, called the retia 

 mirabilia, the type of which is an artery suddenly divided 

 without altering its normal or regular dichotomic disposition. 

 The result of this increase at any one point of the capacity 

 of the vessels is a widened cone and a sudden diminution in 

 the rapidity of the circulation of the blood. This is what 

 takes place in the kidney, as we shall see later, at the level 

 of the vascular pouches, called the glomerules of Malpighi: 

 the effect of this disposition, in slackening the flow of the 

 blood, is to increase the surface of transudation ; this trans- 

 udation takes place under special conditions of pressure 

 (Fig. 48). We find something similar in the system of the 

 venaportae: the blood furnished by the coeliac axis and the 

 mesenteries to the organs of digestion is brought back by a 

 number of veins into a common trunk, called the portal vein, 

 which, instead of emptying immediately into the vena cava, 

 is first distributed in the liver in the same manner as an 

 arteiy, and forms the afferent vessels of the 'liver, the ca- 

 pillaries, and, finally, the efferent vessels, or hepatic veins, 

 which flow into the vena cava. This whole system may be 

 theoretically represented by a cone (Fig. 48) beginning at 

 the trunk of the aorta, and representing the arteries of the 

 intestines, with their capillaries. This arterial trunk is suc- 

 ceeded by a venous trunk, showing the origin and the trunk 

 of the portal vein ; this second cone is followed by a third, 

 arranged in the same manner as an arterial cone (the circu- 

 lation in which is from the top to the bottom), and represent- 

 ing the ramifications of the portal vein in the liver ; at its 



