ELEMENTARY EXPERIMENTAL PHYSIOLOGY 65 



Lesson XI V. To be provided for each pair of Students. 



1. Frog heart apparatus. The end of the recording lever should have a 

 glass writing point, preferably angled and swinging. 2. Recording Drum. 

 3. Time marker giving T \,th second. 4. Needle and thread. 





LESSON XV 

 The Nervous Control of the Heart 



(1) Connect up the apparatus for giving a series of induced shocks 

 (Neef's hammer), inserting a commutator (see Appendix), without 

 crossed wires into the secondary circuit with two pairs of electrodes 

 attached. 



(2) Set a recording drum on the slow gear, and drive from small 

 to large spindle. 



Always fit up the apparatus and test that it is working properly before 

 commencing to prepare the animal. 



(3) Kill a frog and remove the brain in front of the tympanic rings 

 by cutting the head 



across at that level. 

 Then cut the spinal 

 column and cord across 

 between the shoulder 

 blades and pith the 

 lower part, thus leav- 

 ing the medulla oblon- 

 gata isolated and in- 

 tact with the vagi 

 passing from it to the 

 heart. 



FIG. 65. Contraction of the frog's heart. 



The effect of tightening the first Stannius ligature at fin-t 

 gently and then firmly. The curve should be read from 

 right to left. The time is marked in seconds. (L.H.) 



1. Intr a- Cardiac Ner- 

 vous Mechanism. 



Fix one pair of elec- 

 trodes from the com- 

 mutator by means of a 



pin to the cork so that their points touch the crescent, which may 

 be seen as a white crescentic mark between the sinus and auricles 

 on their posterior aspect (Fig. 52), and, with the point of the lever 

 swung off the drum by means of the base -piece, stimulate by closing 

 the key in the primary circuit and opening that in the secondary. 

 If no change in the rate of the heart occurs, increase the strength 

 of the current till a marked change takes place. Do not continue to 

 stimulate after an effect is produced, but at once close the key in the 

 secondary. When the rate of the heart has returned to the normal, 

 swing the lever on and take a tracing as above, marking with a pin 

 upon the drum when the stimulus was applied and when discon- 

 tinued. Let the drum run till the previous rate of the heart is 

 restored. Swing the lever off the drum. What conclusion do you 

 draw from the result obtained ? 



