PECULIARITIES OP BLOOD SUPPLY IN CERTAIN VISCERA 257 



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 matter of fact, we know that the hepatic artery remains constricted 

 as long as the hepatic plexus is stimulated, indicating that the conges- 

 tion of the connective tissue in which the venules lie has become reduced 

 to suchan 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 

 (2) whether this bloodflow can be altered by dilatation or constriction 

 of the vessels brought about through nerves. With regard to the pecu- 

 liarities of bloodflow, it may be stated that there is said to be two periods 

 in each cardiac cycle during which an increase takes place in the mass 



