CHAPTER XVII 

 Cardiac Nerves of Frog 



DESTROY by a wire the spinal cord of a frog, and also remove the 

 cerebral hemispheres ; this can be done without special dissection by 

 cuttingawav with a razor the upper jaw and anterior part of the 

 slnill atthe level of the front of the tympana (see Fig. 77, p. 96). The n 

 posterior part of the brain with the medulla oblongata must not be " 

 injured. Fix a pair of pin-electrodes connected with an induction coil 

 into this part of the skull and arrange for tetanisation of the medulla 

 oblongata using the Helmholtz method (p. 17). Lay the frog upon 

 its back on the frog-cork, and fix it securely by strong pins ; expose 

 tbejieart and the ^hief nerves which are proceeding from the base 

 of the skull to the hyoid region (vagus, glosso-pharyngeal, and hypo- 

 glossal ; see Fig. 65). The vagus gives off a small branch on each side, 

 which runs close along the superior vena cava to the sinus venosus. 

 Place one vagus trunk upon a fine pair of wire electrodes (which must 

 be fixed by plasticine to the frog-cork, not held in the hand), and 

 connect these electrodes, and also those which are fixed into the skull,^ 

 to a commutator without cross wires so that the faradising shocks can 

 be sent to one or other pair as may be desired. Place the frog-cork 

 upon the stand of a frog cardiograph (Fig. 62), and by means of a 

 thread and fine hook attach the apex of the ventricle to the short arm 

 of the light lever. Record the contractions of the heart upon a very 

 slowly moving drum (one revolution in four or five minutes). Use 

 the " stop " for adjusting the lever and for readjusting it after removal 

 from the drum, so that the pressure of the lever point is always 

 exactly the same. Without this precaution the strength of the con- 

 tractions may be wrongly judged, for if the lever is pressing more 

 at one time than at another, the lever will not be raised so high, even 

 if the force of the heart's contractions is unaltered. 



Be careful not to injure the heart more than is absolutely necessary. 

 In order to fix the base securely, pass a strong pin close to the base 

 of the heart and through the vertebral column into the frog-cork. 



The following experiments may be performed upon this prepara- 

 tion : 



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