CHAPTER XVII 



CARDIAC NERVES 



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

 cerebral hemispheres ; this can be done without special dissection by 

 cutting away the upper jaw and anterior part of the skull at the level 

 of the front of the tympana (see fig. 77, p. 95). The 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. 16). Lay the frog upon its back on the frog-cork, 

 and fix it securely by strong pins or rubber bands ; expose the heart 

 and the chief nerves which are proceeding from the base of the skull 

 to the hyoid region (vagus, glosso-pharyngeal, and hypoglossal ; see 

 fig 62). 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 passed through a flat 

 piece of cork (which must itself 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 cardio- 

 graph (fig. 60), and by means of a thread and fine steel 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 contractions 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. 



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