208 EXPERIMENTAL PHYSIOLOGY. 



fine scissors, after which the animal is placed on the left side, so as to expose the 

 structures at the root of the right lung. These are cut through and the operation 

 is repeated on the other side. The aorta must now be made out and cut across 

 just proximal to where its first branch (the innominate) comes off, after which the 

 superior vena cava is cut a short distance from the auricle. A snip is made in the 

 apex of the right ventricle, using a small sharp-pointed scissors for the purpose: 

 this is to permit of free escape of the fluid from perfusing the heart. Finally a 

 thread attached to a packing needle is inserted in the inferior vena cava, and cau- 

 tiously guided upwards so that it emerges from the superior vena cava. The 

 needle is unthreaded, and the thread tied in a loop. This thread is introduced 

 to serve as a guide to the venae cavae into which later a tube is to be inserted. 

 The heart is removed, by cutting the inferior vena cava below the thread, and 

 placed in a dish containing cold Locke's solution. Any blood remaining in the 

 right auricle and ventricle is then washed out by introducing Locke's solution 

 from a large pipette through the superior vena cava. The special cannula is 

 now tied into the aorta, care being taken that no air bubbles are entrapped. 

 (This part of the technique is more conveniently performed with Brodie's appara- 

 tus than with Gunn's, but it is perfectly simple using the latter). A bent pin 

 hook is passed through the outer coat of the right ventricle and a thread attached 

 to it is carried over a pulley to a heart lever arranged to write on a drum. A 

 similar hook and thread may also be attached to the auricle so that simultaneous 

 tracings of auricle and ventricle may be taken. In order to steady the heart it is 

 well to pass a thread through the apex of the left ventricle and tie it below to a 

 glass rod fixed to a stand. 



Demonstration 3. Having secured a tracing of the heart and 

 noted the temperature of the perfusion fluid, the following observa- 

 tions may now be made: 



1. The influence of changing the temperature of the perfusion 

 fluid. This can best be done by adding hot or cold water to the 

 copper vessel connected with the water jacket. 



2. The influence of the local application of heat and cold in the 

 right auricle. To demonstrate this, pass a thin-walled glass tube 

 through the superior and inferior venae cavae using as a guide the 

 string previously placed in the vessels. The upper end of this tube 

 is connected by rubber tubing with a funnel, and either cold or hot 

 water is made to flow through it while a tracing of the beat is 

 being recorded. The temperature of the perfusion fluid is mean- 

 while kept exactly constant (cf. Sherrington). 



3. The effect produced on the beat by increasing the relative 

 proportion of the calcium or potassium in the perfusion fluid. 

 This is very conveniently accomplished in the Brodie apparatus 

 by closing one of the side tubes on the inflow tube leading to the 



