PECULIARITIES IN BLOOD SUPPLY IN CERTAIN VISCERA 267 



about by changes in the caliber of the vessels in the liver itself are 

 partly dependent upon changes in the branches of the hepatic artery. 

 Let us consider briefly how this may be brought about. At the point 

 where the portal and hepatic arteries come together that is, at the in- 

 trahepatic capillaries the pressure of the blood in them must become 

 equal, which means that in its course through the interlobular connec- 

 tive tissue, the branches of the hepatic artery must offer much resistance 

 to the blood flowing through them. This frictional resistance resides in 

 the hepatic arterioles, and since these are richly supplied with constric- 

 tor nerves, great variation in hepatic inflow becomes possible. These 

 changes will affect the degree of tension of the interlobular connective 

 tissue in which the arterioles lie. In this tissue, however, also lie the 

 thin-walled branches of the portal vein. When therefore the tension 

 of this tissue becomes greater, as a result, for example, of vasodilatation 

 in the hepatic artery, the portal vein radicles will become compressed 

 and the bloodflow along them impeded. Conversely, when vasocon- 

 striction occurs in the hepatic arteries, the congestion of the connective 

 tissue becomes diminished, the veins dilate, and the blood flows through 

 them more readily (Macleod and R. G. Pearce 21 ). Experimental evi- 

 dence in support of the above view is furnished by observing the out- 

 flow of blood from the liver before and during stimulation of the he- 

 patic plexus. The first effect is an increase in the outflow, which very 

 soon returns to its original amount, even though the stimulation of the 

 plexus is kept up during the experiment. This return to the normal 

 flow must indicate either that the constriction of the hepatic artery has 

 not been maintained, or that it has been maintained but is accompanied 

 by a compensatory increase in the flow through the portal vein. As a mat- 

 ter of fact, we know from other experiments that the hepatic artery remains 

 constricted as long as the hepatic plexus is stimulated, indicating that the 

 congestion of the' connective tissue in which the venules lie has become re- 

 duced to such an extent, as a result of the constriction, that these open up 

 and permit the blood to flow through them more readily. The initial in- 

 crease in outflow immediately following upon stimulation of the hepatic 

 plexus, is no doubt caused by the squeezing out of the blood already in 

 the hepatic vessels, and it is a result which is often observed in other 

 organs during stimulation of vasoconstrictor nerve fibers. 





 THE CORONARY CIRCULATION 



We have already studied the effect produced on the heartbeat by in- 

 terfering with the flow of blood in the coronary vessels, and it remains 

 for us to study: (1) peculiarities in the bloodflow through them, and 



