238 



PRACTICAL PHYSIOLOGY. 



[XLVI. 



(b.) Prepare N.P. electrodes for a nerve. In this case the electrodes are 

 hook-shaped, and one is adjusted over the other. The upper hooked electrode 

 has a groove on its concavity communicating with the interior of the tube 

 (fig. 162). Place only one plug in the shunt between A and B. 



(c.) Dissect out a long stretch of the sciatic nerve, make a fresh transverse 

 section at both ends, hang it over the upper N.P. electrode (N), and resting 

 with its two cut ends on the lower electrode (C), thus doubling the strength 

 of the current (fig. 162). 



(d. } Remove the plug from C in the shunt and pass the whole of the de- 

 marcation nerve-current through the galvanometer, noting the deflection. 



(<2.) Instead of adjusting the nerve as in (c.), it may 

 be so placed on the ordinary tube N.P. electrodes that 

 the cut end rests on one electrode and the longitudinal 

 surface on the other, thus leaving part of the nerve free. 

 Observe the deflection in this way. 



2. Action-Current of Nerve. 



(a.) Observe the amount of deflection as in (1. .). 

 Stimulate with an interrupted current the free end of 

 the nerve, and observe that the spot of light travels 

 towards zero. This was formerly called the " negative 

 variation " of the nerve-currenC 



3. Electro-Motive Phenomena of the Heart. The 



arrangement of the apparatus is the same as in Lesson 

 XLV. 



(a.) Make a Stannius preparation of the heart, using 

 only the first ligature (Lesson LV. 1) to arrest the 

 heart's action. Lead off with brush N.P. electrodes 

 Fio. 162. Nerve N.P. from base and apex of the quiescent uninjured heart : 

 Electrodes. N. Nerve ; there ig no deflection. 



(b.) Pinch the apex so as to injure it ; it becomes 

 negative ; a difference of potential is at once set up and 

 now the spot of light oscillates with each beat of the heart. 



(c.) Excise a heart so as to get a spontaneously beating ventricle ; lead off 

 from the base and apex of the latter ; observe the so-called " negative varia- 

 tion " with each contraction. 



(d. ) See also Lesson XLVII. 6 for secondary contraction excited by the 

 beating heart. 



4. Capillary Electrometer. 



(a.) Lead off a muscle to the two binding screws of a capillary electrometer. 

 The fine thread of mercury must be observed with a microscope. 



By means of the capillary electrometer Waller has shown the diphasic 

 variation of the heart-current in man and in a living dog. 



C. Clay of electrodes ; 

 Zn. Zincs. 



