

48. Diagram of the double cones of the 

 portal system.* 



PERIPHERIC ORGANS OF THE CIRCULATION. 151 



base (hepatic capillaries) this cone forms a fourth, represent- 

 ing the hepatic veins. In making this passage the blood, 

 therefore, must traverse a 

 system containing twice as 

 many cones as the general (o 

 system, and is subject at 

 each double base (each 

 network of the capillaries) 

 to the slackening which 

 we have mentioned. 



The capillary vessels 

 which are placed in the 

 series of cones of the por- 

 tal vein, moreover, have 

 not the same pressure as 

 the ordinary capillaries. As these systems are not placed at 

 an equal distance from the left ventricle and the right auri- 

 cle, neither can have a pressure hall-way between yj-^ and 

 YQ^ of an atmosphere. The pressure will be less in the 

 hepatic capillaries, because they are nearer the auricle ; and 

 greater in the intestinal capillaries, because they are nearer 

 the left ventricle. The latter condition, as we shall see, is 

 not favorable to the theory of intestinal absorption by simple 

 osmosis. We shall see also that the same theory may be 

 asserted with regard to the capillary system of the kidneys. 



B. Properties and Functions of the Vessels. 



The general conditions of the circulation of the blood, of 

 its pressure and its velocity, which conditions are simply the 

 result of the mechanical arrangement of the blood tubes, 

 may be affected and complicated by the physiological prop- 

 erties of the coats of the vessels, the arteries, capillaries, and 

 veins. 



Arteries. Anatomy teaches us that the arteries are com- 

 posed of three coats or tunics (Fig. 49). Of these three the 

 most interesting to the physiologist is the middle tunic : it 

 contains two essential elements, elastic tissue and muscle 



* The superposition of the two diagrams shows that the pressure is not the 

 same in the capillaries of a portal system and in those of the circulation in 

 general. 



1, General circulation. V, Ventricle. O, Auricle, o, Arteries. 0, Veins. 

 C, Capillaries (pressure 12). 



2, Portal system. V, Ventricle. O, Auricle, a, Arteries. c x c'. First sys- 

 tem of capillaries (pressure 18). SP, Portal trunk, cc, Second system' of 

 capillaries (pressure = 6). v, Vein. 



