AND BLOOD PRESSURE 171 



The arterial wall responds to increased tension by contraction, 

 and to diminished tension by expansion, and thus tends auto- 

 matically to compensate variations in aortic pressure and keep 

 the blood flow constant. That the vaso-motor centres have no 

 share in this is shown by an experiment of Bier. He amputated 

 the limb of a pig, leaving it connected with the body by an arterial 

 cannula only, and found that an active hyperaemia followed a 

 temporary arrest of the arterial supply. 



The following is one of the most interesting experiments of Bier. 

 He produced venous congestion of the arm by means of a bandage, 

 then tightened the bandage so as to obliterate the artery, and 

 lastly after five minutes loosened the bandage. The venous con- 

 gestion was at once replaced by an active arterial hyperaemia. 

 He concludes then while arrest of the arterial flow is followed by 

 active hyperaemia, venosity of the blood causes arterial constriction. 

 To confirm this conclusion Bier rendered bloodless the limb of a 

 pig for ten minutes by means of an Esmarch bandage, and then 

 loosened it. The limb responded by the usual active hyperacmia. 

 He next closed the windpipe of the animal, and noted that the 

 limb first became blue and then paled. On opening the windpipe 

 the active hyperaemia returned, to be abolished once more on 

 closing it. 



One more example of the influence of chemical state of the 

 blood on the circulation is the following. Bier exposed the rabbit's 

 intestine in a bath of warm salt solution, and stopped the blood 

 supply to two loops of it, one of which was empty and the other 

 filled with milk at body temperature. On lessening the ligatures, 

 the one filled with milk responded by an active hypersemia, the 

 other not. Bier says the intestine of a fasting animal does not 

 become hyperaemic after a temporary anaemia, while that of a fed 

 one does, an observation which is of some interest to the surgeon. 

 The stomach of the rabbit does not give the hyperaemic reaction 

 while that of the dog does, so that ligation of the chief arterial 

 supply of the stomach causes necrosis in the one and not in the 

 other animal. Man is like the rabbit in this respect. While 

 obliteration of the mesenteric arteries for a length of only 3 cm. 

 produces necrosis in the small intestine, 10 cm. is the minimum in 

 the colon of the rabbit. 



Bier points out that every organ in functional activity is in a 

 state of active hyperaemia, so too where tissue growth is great as 



