AUTOMATIC CONTRACTIONS OF CARDIAC MUSCLE 265 



and demonstrations can be made on the mammalian heart, but, as this, 

 experiment is usually a demonstration experiment, the detail of procedure 

 is left to be supplied by the demonstrator. 



10. Automatic Contractions of Cardiac Muscle. Isolated portions of 

 the dog's ventricle have been kept in rhythmic contraction by Porter. 

 But the best laboratory material is supplied by the heart of the terrapin. 

 Cut a strip from the ventricle of the terrapin extending around its curved 

 apex, as shown by the dotted line in the accompanying figure, 215. Split 

 this strip longitudinally into two parts, each of which will then be about 

 3 to 4 mm. in diameter by 3 cm. long. Use care to cut smooth strips. 

 Tie a silk thread around the extreme tips of each end of the strip, tying a 

 loop of about i cm. long at one end, and about 10 cm. long at the other. 

 Suspend the strip over a glass hook, figure 216, by the short loop, and con- 

 nect it with a heart lever by the long loop, as shown in the same figure. 

 Use a tension of i gram. Contractions of this strip as arranged will be 

 recorded with a magnification of about five and with the upstroke of the 

 lever, which is convenient for reading and interpretation. Keep the strip 

 moist with physiological saline in a specimen tube i by 3 inches in size. 

 The arrangement of apparatus figured makes it possible easily and 

 quickly to change this solution for any other that may be desired. 



PIG. 215. Heart of the Terrapin to Show the Method of Cutting the Apex Strip. V f 

 Ventricle; Au, auricles; Vc, venae cavae; Ao, aorta. 



Contractions of the ventricular strip in saline begin in from 10 to 

 40 minutes after the preparation is made, and go through a regular sequence 

 of slight increase in rate and amplitude for from 10 to 20 minutes, followed 

 by a very constant rate, but gradually decreasing amplitude for a period of 

 from 2 to 3 hours, figure 171. Saline is used for the ventricular muscle 

 because it increases the rhythmicity to a degree convenient for study. 



