210 INTRODUCTION TO GENERAL PHYSIOLOGY 



be seen forming highly refracting masses of curious shapes where 

 it has escaped from the cut ends of the fibres. To see the axis 

 cylinder, the contents of the medullary sheath may be dissolved 

 away by the action of chloroform. 



Electrical Change in Nerve. The arrangement of apparatus is 

 the same as that for the skin glands (Fig. on p. 198 above). Owing 

 to the high resistance of nerves, the two sciatic nerves of a frog may 

 be laid side by side on a glass slide. One electrode is placed on 

 the surface of the preparation, near the middle, the other one on 

 the cut end, which has been killed for a length of a few mm. by 

 touching with a hot wire. Electrodes for stimulating are placed 

 near the opposite end. The leading-off electrodes are first made 

 to touch each other. If there is any potential difference between 

 them, it is compensated by the slide-wire. It will be small in any 

 case. There will be found to be a fairly high potential difference 

 between the injured end of the nerve and the normal surface, such 

 that the longitudinal surface is positive to the end. This should 

 be absent when two symmetrical places on the longitudinal surface 

 are led off, provided that neither of them is injured. This may be 

 ensured by allowing the nerve to lie in saline solution in the cold 

 for a day before examining it ; all the injured fibres will then be 

 dead. To find which way the spot of light moves when either 

 electrode is positive, attach a bit of zinc to a copper wire from one 

 terminal of the galvanometer and a plain copper wire to the other. 

 On placing the zinc and copper in dilute salt solution, the copper 

 becomes positive to the zinc. The galvanometer must be short- 

 circuited by a low resistance in order to avoid too violent a 

 deflection. 



Having compensated the so-called "current of injury," or 

 " resting current," stimulate by rapid induction shocks with the 

 automatic interrupter of the coil. A deflection will be obtained 

 in a direction opposite to that of the current of injury. 



Nerve Unexcited by Light or Sound Waves. The nerve-muscle 

 preparation is used. No contraction is obtained if a beam of light 

 be allowed to fall on the nerve by placing it in front of the window, 

 screened by a piece of black cardboard, and suddenly removing the 

 screen. Neither will the sound of a bell excite the nerve trunk. 



Receptors of the Skin 



Histological preparations of human skin, showing various forms, 

 may be bought. If fresh skin can be obtained, vertical sections 

 may be made by the method described above (p. 193). Receptors 

 are looked for in the projecting papillae underneath the outer 

 " epidermis." 



