134 COURSE OP THE CIRCULATING BLOOD. 



If that be accomplished, the circulation is restored, and the heart pro- 

 ceeds with its duty. And for these reasons, I believe that in many cases 

 success would be had, where failures are now experienced, if, instead of 

 resorting to atmospheric air, pure oxygen gas or protoxide of nitrogen 

 were administered. 



In the more highly-developed organisms the objects of the circulation 

 are threefold : 1st. To minister to the nutrition of the system ; 2d. To 

 introduce oxygen ; 3d. To remove the products of waste. In man, these 

 various results are accomplished by several different arrangements : 1st. 

 Different ^ e g reater or systemic circulation ; 2d. The less, or pulmo- 

 ciasses of nary circulation ; 3d. The portal circulation ; 4th. The Malpi- 



circulation. g^ circulatioilj &c . 



The course taken by the blood is as follows. Leaving the left ventri- 

 Courseofthe c ^ e of the heart, it passes into the aorta, and is distributed 

 blood in its sys- by the ramifications thereof, constituting the systemic arte- 



temic and pul- . -,-, f .-, T , 1,11 



monaiy circu- nes > to all parts of the system. It moves onward through 

 lations. the capillaries, which may at once be considered as the term- 



inal ramifications of the arteries and the commencing tubelets of the 

 veins. These, converging into larger and larger venous trunks, the sys- 

 temic veins, deliver it into the ascending and descending vense cavas, from 

 which it flows into the right auricle, and from thence into the right ven- 

 tricle of the heart. From thence it is driven into the pulmonary artery, 

 to be distributed to the lungs, and, coming therefrom along the pulmo- 

 nary veins, reaches the left auricle, and from thence it gains the left ven- 

 tricle, which was its starting-point. 



In the pulmonary veins, the left cavities of the heart, and in the sys- 

 Dristribution of temic arteries, the blood is crimson. In the systemic veins, 

 crimson and of the right cavities of the heart, and pulmonary artery and its 

 branches, it is blue. The change from crimson to blue takes 

 place in the systemic capillaries, and from blue to crimson in the pulmo- 

 nary. The systemic, or greater circulation, is considered as beginning 

 at the left ventricle and ending at the right auricle ; the pulmonary, or 

 less circulation, begins at the right ventricle and ends at the left auri- 

 cle. This double course is sometimes, among authors, illustrated by 

 likening it to the figure 8, the upper loop representing the pulmonary, 

 the lower the systemic circulation, and the heart placed at the nodal point. 



As has just been remarked, there are other subordinate circulations, 

 The portal but of these only one need attract our attention at present it 

 circulation. j s tne portal. This originates in a system of capillaries, the 

 veins belonging to the digestive apparatus, which, converging rapidly to- 

 gether, form a common trunk, the portal vein. This at once ramifies 

 like an artery in the substance of the liver. From the resulting capilla- 

 ries, the portal blood passes into the commencing capillaries of the hepat* 



