270 EXPEiaMENTAL IXVKSTICATIOX OF THE ACTION OF MEDICINES. 



iiieiit, the lymph produced, or the secretion poured forth ; 

 and by comparin,<]f the results when the drug was added with 

 those obtained when it was witldield, we may, I think, gain 

 such a knowledge of its action as could be got in no other 

 way. 



Scliema of the Circulation. — In the livdng body, a constant 

 stream of blood is kept up in the vessels, in exactly the same 

 way that a constant current of air is produced in Eichardson's 

 spray-appaiatus. By removing the glass or metal lube from 



Fia. 132. 



Simple schema of the circulation, consisting of a spray-producer, bladder, and 

 mercurial manouiettr. The elastic ball represents the heart; the elastic 

 bag, covered with netting to prevent too great distension, represents the 

 aorta and arterial system ; and the bladder represents the venous system. 



one of these, and attaching a nozzle with a small stopcock to 

 the india-rubber tube in its stead, we obtain a very good schema 

 of the circulation ; and, by imitating on it the changes which 

 occur in the heart and vessels, we may form a much clearer 

 idea of them than we could otherwise do. The india-rubber 

 ball will represent the heart ; the elastic bag, surrounded by 

 netting, will represent the elastic aorta and larger arteries ; and 

 the stopcock, which regulates the size of the aperture through 

 which the air escapes, will represent the small arteries and 

 capillaries, whose contraction or dilatation regulates the flow of 

 blood from the arteries into the veins. If we turn the stop- 



