EXAMINATION OF CAPILLARIES. 317 



Fio. 140. 



by connecting a sphygmoscope with a schema of the circulation 

 such as I have already described, and compressing the india- 

 rubber ball which represented the heart with greater or less 

 force and suddenness, care being taken, however, to empty it 

 completely each time, so that the amount of air sent out should 

 always be alike. 



As variations in the blood-pressure may be due to alterations 

 in the activity of the heart or the size of the capillaries, or to 

 both together, we cannot say when it is due to the one and 

 when to the other, unless we can keep one of them constant 

 while we allow the other to alter, or unless we examine them 

 both separately. 



Elimination of the Action of the Heart. — We may keep the 

 action of the heart tolerably constant, and thus ascertain with 

 considerable exactitude the action of any drug on the exit-tubes 

 — whether they be arterioles or capillaries matters not — by 

 separating the heart from the nerve-centres, and then injecting 

 the drug into the circulation. 



Division of Cardiac Nerves. — This separation can be effected 

 to a considerable extent by dividing the sympatheti.es, vagi and 

 depressors in the neck ; but it is done much more effectually by 

 dividing the nerves near their entrance into the heart by a fine 

 wire heated by means of a galvanic battery. 



As poisons generally produce their most marked effects on 

 the heart of mammals through the nervous centres whose con- 

 nexion with the heart we have thus severed, alterations in the 

 blood-pressure will be due to changes in the vessels, except in 

 so far as the drug may have affected the cardiac muscle or 

 ganglia. But, just as we obtained the most exact results when 

 we examined the heart altogether apart from the blood-vessels, 

 so we shall probably come to the most satisfactory conclusions 

 regarding the vessels by observing them apart from the heart. 



