CHAP. XXVIII.] PORTAL CIRCULATION. 347 



The cause of this difference in the frequency of the pulse in 

 different postures resides probably in the effort employed to main- 

 tain the muscular contractions necessary to support the erect or 

 sitting postures. But careful experiments are still wanting to as- 

 certain whether a simple difference of posture, without muscular 

 exertion, would develop a change in the frequency of the pulse. 

 Those as yet done by the revolving board, seem to have had refer- 

 ence only to the exertion of muscular force in the production of 

 the change of posture, and not to that required for the continued 

 effort to maintain the attitude. 



The Course of the Circulation in the Adult. Taking the left 

 ventricle as the starting point for the circulation, we may describe 

 the blood as pursuing the following course. By the left ventricle 

 it is driven through the aorta into every artery of the body save 

 the pulmonary; and having passed through the capillary system 

 it enters the venous radicles, and from them it passes to the venous 

 trunks ; it is at length returned by two great trunks, the superior 

 and inferior vence cavce to the right auricle of the heart. This por- 

 tion of the circulation, from its traversing the whole system, except 

 the lungs, and from its occupying by far the largest part of the 

 body, is called the systemic or greater circulation. The venous 

 blood, brought by the great venous trunk to the right auricle, is 

 expelled by that cavity into the right ventricle, which drives it 

 by the pulmonary artery through the lungs to the pulmonary veins, 

 through which it passes to the left auricle, and so on to the left 

 ventricle. This portion of the circulation, traversing only the lungs, 

 and connecting the right ventricle and left auricle, forms the lesser 

 or pulmonic circulation. 



Of the Portal Circulation. In general the arterial blood passes 

 through a single system of capillaries and veins before it is returned 

 to the auricle. But there are two remarkable exceptions to this 

 one in the portal circulation of the liver, the other in the 

 kidneys. In both these cases, the blood passes through two sub- 

 systems of capillaries after it leaves the arteries. Thus, as regards 

 the hepatic circulation, the blood conveyed to the intestines by the 

 arteries, passes through the intestinal capillaries into the intestinal 

 veins, whence it passes to the trunk of the vena portse, which 

 again transmits it to the hepatic capillaries, and thence to the 

 hepatic veins, through which it reaches the heart. A portion of 

 the circulation, of which the chief vessel is formed like a vein, and 

 distributes its blood like an artery, is called a portal circulation. 

 A similar circulation is found in the kidneys. The afferent arteries 



A A 2 



