228 EXPERIMENTAL PHYSIOLOGY. 



midway between the two, place a Gaskell's clamp (Fig. 106, 

 C) fixed in an adjustable arm attached to the same stand. 



To support the upper 

 lever, fix to it a fine 

 thread of caoutchouc (E), 

 and attach the latter to a 

 slit or other arrangement 

 on the top of the support. 

 The clamp consists of two 

 fine narrow strips of brass, 

 like the points of a fine 

 pair of forceps, which can 

 be approximated by means 

 of a screw. 



(b.) Expose the heart 

 of a pithed frog. Tie a 

 fine silk thread to the 



Fig. 106. -Gaskell's Clamp. apex of the ventricle, and 



another to the upper part 



of the auricles, and excise the heart. Tie the auricular 

 thread to the upper lever and the ventricular one at a 

 suitable distance to the lower lever. 



(c.) Adjust the clamp (Fig. 106, 0) so as to clamp the heart 

 in the auriculo-ventricular groove, but at first take care not 

 to tighten it too much, or merely just as much as will sup- 

 port the heart in position. After fixing the heart by means 

 of the clamp, fix the two levers so that both are horizontal, 

 and adjust the caoutchouc thread attached to the upper one, 

 so that it just supports the upper lever, and when its elas- 

 ticity is called into play by the contracting auricles pulling 

 down the lever, it will, when the auricles relax, raise it to 

 the horizontal position again. 



(d.) Adjust a time-marker to write exactly under the 

 writing-points of the two levers. Moisten the heart from 

 time to time with serum or dilute blood. 



(e.) After obtaining a tracing where the auricle and ven- 

 tricle contract alternately, screw up the clamp slightly until 

 the ratio of auricular to ventricular contraction alters i.e.* 

 until by compressing the auriculo-ventricular groove, the 



