100 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



lateral movement whenever a current is passed through it. The instrument 

 may be made of great delicacy so as to detect very minute currents, and, 

 moreover, it has the very great advantage of responding accurately to rapid 

 changes in potential. If the shadow of the thread is allowed to fall upon 

 sensitized paper properly adjusted upon a rotating surface, its movements may 

 be photographed and a permanent record be thus obtained (see Fig. 22 for 

 an example of such a photographic record showing the electrical changes in 

 a contracting muscle). 



The Capillary Electrometer. The principle of the construction of 

 the capillary electrometer is illustrated in Fig. 41. A glass tube, a, is drawn 

 out at one end into a very fine capillary, the end of which dips into some 

 diluted sulphuric acid contained in the vessel (/). At the bottom of this 

 vessel is a layer of mercury connecting with a wire, g, fused into the glass 

 vessel. The tube a is partially filled with redistilled mercury, which pene- 

 trates for a short distance into the capillary. By means of pressure applied 

 from above c, the mercury can be forced through the capillary. Then by 

 diminishing the pressure the mercury can be brought back into the capillary 

 a certain distance, drawing after it some of the dilute 

 sulphuric acid. The mercury in tube a is connected 

 with the other pole of the battery by a wire fused into 

 its wall and dipping into the mercury. By regulating 

 the pressure on the mercury the point of contact be- 

 tween the thread of mercury and the sulphuric acid 

 in the capillary, d, can be brought to any desired 

 position. An equilibrium is then established which 

 will remain constant as long as the conditions are not 

 changed. If now the circuit from a battery or other 

 source of electricity for example, the excised nerve 

 or muscle is closed, the current entering by wire g, 

 if this represents the anode, traverses the sulphuric 

 acid and mercury in the capillary and returns by the 

 wire h. At the moment of the establishment of the 

 current the equilibrium of forces that holds the mer- 

 cury at a certain point in the capillary is disturbed, 

 the end of the mercury thread moves upward with 

 the current for a certain distance, depending on the 

 strength of the current and the delicacy of the capillary. 

 If the current be passed in the opposite direction the 

 mercury will move downward a certain distance. The 

 meniscus of contact moves up or down with the direc- 

 tion of the current, owing, it is supposed, to a change 

 in the surface tension at this point. The capillary tube 

 as used for physiological purposes is too small for the 

 movements of the mercury to be detected with the eye. 

 It is necessary to magnify it either with a microscope 

 or a projection lantern. Ordinarily the electrometer 

 is so made that it can be placed upon the stage of the 

 microscope and the capillary be brought into focus 

 at the meniscus, as shown in d, Fig. 41. By means of 

 proper apparatus the movement can be photographed 

 and thus a permanent record be obtained of the direc- 

 tion and extent of movement of the mercury. 

 Non-polarizable Electrodes. In connecting a muscle or nerve to an elec- 

 trometer or galvanometer it is necessary that the leading off electrodes 

 that is, the point of contact between the wires and the muscle or nerve 

 shall be iso-electric and non-polarizable. By iso-electric is meant that the 

 two electrodes have the same electrical potential, and it is obvious that the 

 leading off electrodes must fulfil this condition approximately at least, since 

 otherwise the current obtained from the muscle or nerve could not be attrib- 

 uted to differences in potential in the tissue itself; it would be shown by any 

 other moist conductor connecting the two electrodes. Two clean platinum 

 electrodes would fulfil this condition. A more serious difficulty is found in 



i 



Fig. 44. To show 

 the structure of a non- 

 polarizable electrode : 

 1, The pad of kaolin or 

 filter paper moistened 

 with physiological sa- 

 line (NaCl, 0.7 per 

 cent.) (this is placed on 

 the tissue) ; 2, the sat- 

 urated solution of zinc 

 sulphate; (3) the bar 

 of amalgamated zinc. 



