318 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



Yon will remember that, during the diastole, the circulation is 

 carried on entirely independently of the heart by the pressure 

 of the blood in the arteries ; and, if we can prolong the diastole 

 sufficiently, we shall be able to tell whether the vessels are 

 dilated or contracted by simply seeing whether the pressure 

 sinks quickly or slowly. 



If we prevent any blood from being pumped into the aorta 

 by the heart, the arterial system will come to resemble a bottle 

 with a hole in it, from which the fluid which it contains is 

 running. The larger the hole, the more quickly will it run out 

 and the bottle become empty, and vice versd ; and, in the same 

 way, the more dilated the capillaries are, the quicker will the 

 blood run out of them into the vein, and the pressure sink in 

 the arteries ; the more contracted the capillaries are, the more 

 slowly will the blood flow through them, and the more gradual 

 will be the fall of pressure. In the case of many poisons, we 

 may do this by irritating the vagi' before poisoning, and seeing 

 how quickly the pressure falls while the heart is standing still ; 

 and then repeating the experiment after injecting the poison. 

 If the pressure fall more quickly in the second case, we know 

 that the vessels have become dilated ; and if more slowly, that 

 they have contracted. Of course, only those parts of the 

 tracings in whicli the pressure has been the same are to be 

 compared with each other; but, if we stop the heart long 

 enough, we can always get parts in both which are capable of 

 comparison. 



Y/hen the poison paralyses the vagas, as atropia does, this 

 method fails ; and then we must open the thorax, perform arti- 

 ficial respiration, and put a ligature round the aorta. 



Artificial Circulation in Afaminals. — As an animal quickly 

 dies when the aorta is ligatured, it is better to carry on artificial 

 circulation by a syringe through a cannula inserted into the 

 aorta, as Herino: has done in his researches on the con- 

 nexion between arterial movement and respiration. After the 

 blood has circulated once, it may be defibrinated, shaken with 

 air, warmed to 40° Cent., and re-injected. Instead of using a 

 syringe, the cannula in the aorta may be connected with the 

 nozzle M, Fig. 141, and the blood put in the flask E. It can thus 



