352 GENERAL DIRECTIONS. 



Moistening either muscle or nerve, and especially the nerve, 

 even with Na. 01. .75 p. c. is undesirable, as it tends to intro- 

 duce errors. It is necessary, therefore, so to place the nerve 

 and muscle that they may be experimented upon in an atmo- 

 sphere kept uniformly damp. This is effected by means of 

 the moist chamber (fig. 270). 



This consists of a platform of hard wood or ebonite, which 

 slides up and down, and can also be turned from side to side 

 and clamped in any position, on an upright. Let into the 

 platform are two or more pairs of insulated binding screws for 

 receiving the various wires for the electrodes, as well as 

 clamps into which the leaden electrode bearers are fixed. The 

 upright on which the platform slides also carries above a 

 sliding arm, with a clamp for holding the femur of the nerve- 

 muscle preparation, and below a similar sliding arm to which 

 the lever is fixed. The attachment of the muscle to the lever 

 is carried through a slit in the platform. A common glass 

 shade, fitting into a rim in the platform, covers everything ; 

 and when several pieces of wet blotting-paper are placed inside 

 the cover, the atmosphere within ma} r be kept saturated with 

 moisture for any length of time. 



IV. Nerve Chamber. When the phenomena of electro- 

 tonus (Chap. XXVII.) are being studied, it is very desirable 

 to have a smaller chamber than the ordinary moist chamber to 

 work in. This may be gained by having a small glass trough, 

 about three inches long and one broad, with a movable top, 

 and the glass of one of the sides replaced by a piece of India- 

 rubber sheeting with a slit along the middle. The electrodes 

 may be introduced through the slit at the side (the India- 

 rubber closing on them), the nerve placed in position on the 

 electrodes, a few morsels of wet blotting-paper inserted (so as 

 not to touch the nerve), and the cover laid on. The nerve 

 may thus be kept from drying for a considerable time. 



V. Electrodes. For many purposes the ends of the 

 copper wires may be used without any special arrangement. 

 The two wires may be kept separate, or they may be fixed at a 

 definite distance from each other in an insulating handle of 

 bone, wood, gutta-percha, etc. (fig. 271). It is often con- 

 venient to have the ends of wires completely covered, except 

 just at one point in each to which the nerve may be applied 

 (fig. 271). In this case it is also frequently an advantage to 

 have the ends somewhat curved. Such a pair of electrodes 

 can easily be made at once by fastening two wires, bent as 

 desired, on either side of a slip of wood, or other non-conduct- 

 ing material, of the thickness required to separate the wires 

 sufficiently, coating the whole with melted paraffin, and, when 

 the paraffin has cooled, scraping a little away at one spot till 

 a point of each wire is exposed. Platinum wire, or slips of 



