PECULIARITIES IN BLOOD SUPPLY IN CERTAIN VISCERA 265 



any pulmonary congestion under the above conditions. When moderate 

 or slowly increasing resistance is offered to the systolic discharge from 

 the left ventricle, no back pressure effects develop in the pulmonary veins 

 (again in obedience to "the law of the heart"), but if the aortic resistance 

 becomes so high that the ventricle dilates beyond its physiological capac- 

 ity and fails to discharge its normal volume, then pulmonary venous con- 

 gestion develops. But it may not be till much later that the "back 

 pressure" shows itself in the pulmonary artery and this has been ingeni- 

 ously explained as being due to a diminution of the capacity of the right 

 ventricle, and therefore of the discharge of blood from it, because of 

 bulging of the intraventricular septum. When the above pressure differ- 

 ences become persistent, and especially when the myocardium begins to 

 break down, various pathological disturbances of the circulation (cardiac 

 decompensation) supervene as described in the various textbooks on 

 clinical medicine. 



CIRCULATION THROUGH THE LIVER 



The liver is the only gland in the body receiving both venous and 

 arterial blood, the former being supplied to it at a very low pressure 

 by way of the capacious portal vein, and the latter at very high pressure 

 by the strikingly narrow hepatic artery. Except for the relatively 

 small amount of blood which is supplied to the walls of the blood vessels 

 and the biliary ducts, none of the hepatic artery blood mixes with that of 

 the portal vein until the vessels enter the hepatic lobules. Beyond this 

 point the two blood streams mix and the combined stream is drained 

 away by the sublobular and hepatic veins. 



Methods of Investigation 



To study the relative importance of these tAvo sources of blood supply, and also to 

 .nvestigate th<? manner in which the latter is controlled, the most satisfactory method 

 has consisted in measurements of changes in volume flow rather than in those of 

 changes in pressure. The volume-flow measurement has been made either by connecting 

 stromuhrs (page 207) to the hepatic artery or portal vein, or by measuring the out- 

 flow of blood from the hepatic vein into the vena cava, first with both inflow vessels 

 intact, and then with one of them ligated. An objection to the first (the stromuhr) 

 method is the possible interference with bloodflow or blood pressure produced by 

 inserting the stromuhr into the entering vessels, and also the fact that simultaneous 

 measurement of the flow in both vessels cannot be made satisfactorily. 



To measure the outflow from the hepatic veins, the aorta is ligated below the celiac 

 axis and a wide cannula is inserted into the central end of the vena cava below the 

 level of the liver, a loose thread being placed around this vessel just above the dia- 

 phragm. By pulling on this thread the vena cava becomes obliterated, and the blood 

 from the hepatic veins is therefore diverted into the cannula, through which it flows 

 into one end of a vessel shaped somewhat like a sputum cup (the receiver), the other 



