THE GENERAL CIRCULATION. 



89 



polish, and, according to Henle, an epithelium composed 

 of vesicles and scales, analogous to those of serous and 

 mucous membranes. 



The portal vessels are veins, but differ from other veins 

 in not having any valves. They are endowed with the 

 properties of contractility, extensibility, and elasticity. 



Their function is to convey venous blood, of the darkest 

 kind, from the abdominal digestive organs, except the liver, 

 to this gland, where the bile is separated from it. The 

 ductus venosus is the connecting link between the portal 

 and the next or general circulation. 



THE THIRD OR GENERAL CIRCULATION. 



The general circulation is styled the true, 

 or Circulation Proper. 



It commences in the left ventricle of the 

 heart by the aorta, extends and ramifies 

 through every part of the body, and then 

 returns, by the vena cava, back again to the 

 heart upon its right side, thus completing 

 the circle. From this mode of distribution, 

 we have the/orm of two trees one the aorta, 

 the other the vena cava the former carrying 

 the blood from the centre to the circumfer- 

 ence, the latter bringing it back changed, 

 from the circumference to the centre. The 

 heart is the centre. 



The heart, arteries and veins, then, com- 

 prise the three natural divisions of the gen- 

 eral circulation. 

 M. Bichat makes two great divisions of this circulation. 



1st. The vascular system of red blood. 

 2d. The vascular system of black blood. 



The first division commencing in the lungs, with the red 



FIG. 7 represents the blood moving in a circle. 1, 1, Superior and Inferior 

 Cava. 2, Right Auricle. 3, Left Ventricle. 4, 4, Aorta and common Carotid 

 Artery. 



