116 EXPERIMENTAL PHYSIOLOGY 



If the heart be kept much longer at this temperature, or for a 

 shorter time at a higher temperature, heat-contraction is more pro- 

 nounced, becomes complete, and then no recovery occurs on cooling. 



The effect of heat and cold upon the heart in situ. — The effect upon 

 the beat of raising or lowering the temperature of the whole heart has 

 been determined by several methods. We can employ the same 

 arrangement as that of the previous experiment, but having pithed 

 the brain it is best to curarise the frog. It is then pinned down to 

 the cork and the heart attached to the lever. The beaker of fluid is 

 then brought up as before, allowing the legs to remain out of the 

 beaker. The results obtained by this method are practically the same 

 as those already described above. 



Engelmann passes a tube through the oesophagus and out of an opening 

 in the stomach wall. Water of different temperatures is then circulated 

 through the tube. 



Another simple plan is to arrange the heart for recording by the suspen- 

 sion method, and then while the tracing is being taken a fine stream of 

 normal saline at the temperature required is directed on to the heart through 

 a glass tube drawn out to a fine capillary orifice. By this method it is 

 possible to limit the cooling or heating mainly to the auricles and sinus 

 without affecting the ventricle. 



A third plan is to record by the suspension method, having previously 

 arranged a coil of platinum wire around the heart. The coil is then heated 

 by an electrical current, and thus forms a small source of heat, whose action 

 may, by altering its position, be limited chiefly to either the sinus and 

 auricles or to the ventricle. 



The results given by these methods are similar to those already 

 given for the excised heart, but with the difference that where the 

 alteration of temperature is limited chiefly to the sinus, the result 

 tends towards alteration in rhythm without alteration in the character 

 of each beat. On the other hand, where the heating or cooling effect 

 is chiefly localised to the ventricle the main effect is an alteration in 

 the force and character of the beat with no change in the rhythm. 

 As in the first experiment, when the rhythm alone is altered, heat 

 tends to accelerate ; when the character is altered, heat up to about 

 20° tends to increase the force, above that temperature to decrease the 

 force, until at about 30° the rhythm is stopped, though on again cool- 

 ing the heart will recover. At about 35° C. the heart begins to pass 

 into the state of heat-rigor, and no complete recovery is then possible. 



THE EFFECT OF HEAT AND COLD UPON A SINGLE 

 CONTRACTION OF THE VENTRICLE 



In addition to examining the result of varying the temperature 

 upon a heart beating rhythmically the alteration in a single beat should 



