LESSON XV 



THE HEART (Continued) 



STANNIUS' EXPERIMENT. STAIRCASE PHENOMENON. SUMMATION 

 OF STIMULI. ACTION OF THE CONSTANT CURRENT, ETHER, AND 

 CHLOROFORM. DISSECTION OF THE MAMMALIAN HEART 



1. Stannius' Experiment. Pith a frog and expose the heart by a 

 median incision through the wall of the thorax. Use two thin silk 

 threads. Place one around the sino-auricular groove, and tighten it 

 moderately until the auricles and ventricle cease beating. Apply a 

 second ligature to the auriculo ventricular groove, and tighten it suf- 

 ficiently until all three parts of the heart beat again. Explain this 

 phenomenon. 



2. Staircase Phenomenon. Remove the ligature previously applied 

 to the auriculoventricular groove, so as to render the auricles and ven- 

 tricle again quiescent. Connect the latter with a light heart lever 

 (suspension method), and stimulate its substance with single induction 

 shocks at intervals of five seconds. Each time move the stationary 

 drum a short distance. The heart usually reacts to stimuli of different 

 strengths by giving maximal contractions, but its amplitude of reaction 

 is determined chiefly by its condition. In the Stannius preparation 

 certain conditions have arisen which convert the first four or five con- 

 tractions into an ascending series. 



3. Summation of Stimuli. The Stannius preparation may also be 

 made to show the phenomenon of summation of stimuli. To accom- 

 plish this end separate the secondary coil from the primary until the 

 break shock just ceases to be effective. Stimulate the heart with this 

 subminimal stimulus in quick succession until a contraction is obtained. 



4. Incomplete Tetanus. The Stannius preparation may also be 

 made to yield an incomplete tetanus by passing a quickly interrupted 

 current through it. 



5. Effects of the Constant Current. Remove the ligature which 

 has previously been applied to the sino-auricular groove. The heart 

 should resume its normal beat. If not, use a fresh preparation. Insert 

 in the frog's mouth the wire connected with the zinc of a dry cell 

 (cathode). Place the flat end of the wire attached to the carbon 

 (anode) upon the surface of the ventricle. As the well-filled ventricle 

 contracts, that portion of it which rests against the anode will remain 

 relaxed and present a flushed appearance. In this way a local diastole 

 is produced in a general field of systole. Suddenly remove the wire, 

 breaking the circuit. The area just alluded to now remains contracted, 

 and shows, therefore, a pale appearance. A local systole arises within 

 a field of general diastole. This experiment may be employed to show 

 that the anode depresses on the make, but stimulates on the break. 



