ANIMAL AND VEGETABLE 247 



moist substances, and their conductivity, instead of their 

 resistance, increases in a physiologically defined ratio with 

 rise of temperature, while the electromotive force fluc- 

 tuates during certain periods of the twenty -four hours and 

 also in accordance with the degree of fatigue to which the 

 patient has been subjected. It will be seen, therefore, that 

 while R may be constant, neither E nor C can be said to be 

 so. For this, if for no other reason, the hand-to-hand 

 deflection must be carefully taken. When this is done all 

 the body deflections must, by Ohm's law, be in conformity 

 with it. 



Another point which calls for consideration is the 

 capacity of our condenser-ganglion cells and condenser- 

 compartment muscular fibres. We have seen that a 

 capacity of one Farad with a quantity of one coulomb 

 causes a potential of one volt, and the fact that we have to 

 go into minute fractions of each unit does not affect the law. 



The potential at any point (supposing R to be constant) 

 is liable to variation by any difference in E (producing a 

 difference in C), while a rise or fall of temperature may not 

 only alter the resistance of R, generally or locally, but also 

 the internal resistance (r) of all or some of the cells. 



Care, then, must be taken when galvanometric examina- 

 tions are made to observe the temperature of different parts 

 of the body, as one part may be colder than another, and by 

 giving a subnormal deflection introduce error into diag- 

 nosis. Furthermore, the utmost vigilance must be ob- 

 served to ensure the conditions of contact being equal, as, 

 if one part of the skin is more moist than another, the 

 result, generally speaking, will be a higher deflection from 

 that part. Inversely the presence of fat in the skin and 

 subcutaneous tissue would tend to interpose resistance and 

 therefore diminish the deflection, etc. 



We may now proceed with our illustration. When 

 any amount of resistance is introduced between the 



