AUTOMATIC CONTRACTIONS OF THE CARDIAC MUSCLE 231 



heart, but, as this experiment is usually a demonstration experiment, the detail 

 of procedure is left to be supplied by the demonstrator. 



9. Automatic Contractions of the Cardiac Muscle. Isolated por- 

 tions 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 terra- 

 pin. Cut a strip from the ventricle of the terrapin extending around its 

 curved apex, as shown by the dotted line in the accompanying figure, 214. 

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

 3 to 5 mm. in diameter. Use care to cut smooth, straight 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 locm. long at the other. Suspend the strip 

 over a glass hook, figure 215, by the short loop, and connect it with a heart 

 lever by the long loop, as shown in the same figure. Use a tension of one 

 gram. Contractions of this strip as arranged will be recorded with a mag- 

 nification of about five and with the upstroke of the lever, which is convenient 



FIG. 214. Heart of the Terrapin to Show the Method of Cutting the Apex Strip. V, Ventricle; 

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



for reading and interpretation. The strip may be kept moist with physio- 

 logical saline in a specimen tube of about i by 3 inches in size, and the 

 arrangement of apparatus figured makes it possible easily and quickly to 

 change this solution for any other that may be desirable. 



Contractions of the ventricular strip in saline usually 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. 



This preparation makes possible many instructive experiments tending 

 to show fundamental properties of cardiac muscle. The preparation con- 



