112 GENERAL PROPERTIES OF LIVING TISSUES 



Cover the indifferent electrode (consisting of a 

 brass plate furnished with a binding post) with 

 cotton wet with saline solution. Mount a non- 

 polarizable boot electrode. Con- 

 nect a dry cell with the metre 

 posts (0 and 1) of the rheochord 

 through a simple key (Fig. 28). 

 Connect post and the slider 

 through a pole-changer (with 

 cross-wires) with the electrodes. 

 Expose the heart, according 

 to the following method : 

 Place the brainless frog, back down, in the 

 holder (Fig. 29). Cut through the skin across 

 the middle of the body from side to side. 

 Make a second cut in the middle line from the 



Fig. 29. The frog board, with spring clips ; about one-fourth the actual 

 size. 



first cut to near the lower jaw. Turn back the 

 flaps. Cut through the sternal cartilage near its 

 lower end, thus avoiding the epigastric vein. 



