LABORATORY WORK 20; 



closing a key in the primary circuit, and also by using the automatic 

 interrupter, which has a much more rapid rate of vibration. 



Heart Muscle 



There are certain facts which require delicate and exact 

 apparatus to demonstrate on voluntary muscle, but can more 

 easily be observed on the slowly contracting heart muscle of 

 the frog. 



It is necessary to be able to make use of a frog heart which 

 does not beat spontaneously. This is attained by the application 

 of a ligature in such a way as to cut off the sinus from the rest of 

 the heart (Stannius' ligature). Pass a thread between the aortas 

 and the auricles as described above (p. 188). Turn the heart 

 forwards. Bring the ends of the thread around the auricles and 

 tie so that the knot presses on the place where the auricles join 

 the sinus, marked by a whitish line. In addition to cutting off 

 the impulses from the sinus (see below, p. 225), it is probable that 

 the vagus nerves are stimulated also. The stoppage of the heart 

 does not last long as a rule, so that the experiments on it must 

 be done as quickly as possible. Have everything ready before the 

 application of the ligature. Cut out the heart and fix it to the 

 cork plate by a pin through' the aortic bulb. Pass a tiny bent pin 

 through the apex of the ventricle and connect it by a thin thread 

 to the lever, which is fixed above the heart. If the same lever as 

 that used in previous experiments with the heart be used, there 

 must be a prolongation to the opposite side of the axis, since the 

 contraction of the heart pulls downwards in the present case. 



The " Staircase" This requires that the muscle should have 

 rested for some time, so that it should be the first experiment 

 made. 



Make electrodes of fine wire, the two wires being attached by 

 sealing wax at about 2 mm. apart, leaving about a centimetre free 

 at the end. Arrange these so that they touch the ventricle gently, 

 supporting them on" a lump of plasticine. Arrange the stimulating 

 coil so that single shocks can be given by a key in the primary 

 circuit. Do not use shocks stronger than can be comfortably felt 

 on the tongue. Allow the tracing point of the lever to rest lightly 

 in contact with a thinly smoked surface, which is at rest. Give a 

 series of stimuli, each following the other just after the effect of 

 the previous one is over, and moving the surface by hand a few 

 millimetres between each. The height of the contractions will 

 increase for the first few beats and then remain stationary. 



" All-or-Nothing" When the steady stage has been reached, 

 vary the strength of the stimulus. It will be found that the beats 



