ELECTRO-PHYSIOLOGY, PHYSICAL CONSIDERATIONS. 639 



bodies is dependent upon the size of the surfaces in contact. The fluids, for 

 example the solutions of acids, alkalies or salts, are designated exciters of electricity of 

 the second class. They form no definite tension-series among themselves. Immersed 

 in most of the fluids named, the metals nearer the positive side of the tension- 

 series, particularly zinc, prove in greatest degree electrically negative, while those 

 situated nearer the negative side are so in less degree. 



If two different substances of the first class be immersed in a fluid, with- 

 out coming in direct contact, for example zinc, and copper, free negative electricity 

 will appear at the projecting extremity of the (positive) zinc, and free positive 

 electricity at the projecting extremity of the (negative) copper. Such a combina- 

 tion of two electromotors of the first class with an electromotor of the second 

 class is designated a galvanic circuit. As long as the two metals remain separated 

 in the fluid, the circuit is said to be open; as soon, however, as the projecting 

 extremities are connected, for example by a wire arc, the circuit is closed and 

 a galvanic current results. Both forms of electricity flow mutually into and neutral- 

 ize each other, although in accordance with the degree in which the tensions 

 neutralize each other new electricity is constantly generated in the circuit. 



The galvanic current encounters resistances in its course that are designated 

 conduction-resistance (W). This is (i) directly proportional to the length (1) 

 of the circuit; (2) inversely proportional to the transverse section of the circuit 

 (q) , the length being the same; and (3) dependent upon the molecular peculiarities 

 of the materials (specific conduction-resistances}. Therefore, the conduction resist- 

 ance W = (s, 1) -i- q. 



The conduction-resistance increases in the case of metals and diminishes in 

 the case of fluids with increase in temperature. 



The strength of the galvanic current (S), or the quantity of electricity passing 

 through the closed circuit, is thus proportional to the electromotive force (E), or 

 the electrical tension, but inversely proportional to the total conduction-resist- 

 ance (L). Therefore, S = E -H L (Ohm's law). 



The total conduction-resistance in the closed circuit, however, is made up 

 (i) of the resistance in the closing arc, external resistance, and (2) of the 

 resistance within the battery itself, internal resistance. The specific conduction- 

 resistance of the different substances is thus variable. In the case of metals it 

 is relatively slight, in that of fluids, however, quite marked. The specific con- 

 duction-resistance or rather the specific conductivity is at present generally indi- 

 cated with reference to mercury as the unit. Accordingly, the conductivity of 

 copper is 55, that of iron from 6 to 10, that of German silver from 3 to 6. In 

 the case of fluids the resistance is exceedingly slight for concentrated salt-solution 

 0.00002, for concentrated solution of copper sulphate 0.000004. 



Conduction in Animal Tissues. In animal tissues the conduction-resistance 

 is exceedingly great, generally some millions of times as much as in metals. A 

 constant current passing from the skin through the animal body encounters 

 progressively diminishing resistance, on account of the galvanic conductivity of 

 the water in the epidermis and the increased fulness of the vessels in consequence 

 of the cutaneous irritation. Nevertheless, different portions of the surface of the 

 body react differently, the least resistance being offered by the palms of the hands 

 and the soles of the feet. The seat of the resistance is the epidermis, after removal 

 of which, as by a cantharidal blister, the resistance is greatly reduced. The 

 resistance is diminished, however, by increased superficies of the electrodes, and by 

 increased moisture, heat, and intensity of their saturation. It is greatest in the 

 extremities, least in the face. Dead tissue is usually a poorer conductor than 

 living tissue. If the current is passed transversely through a muscle, it encounters 

 nine times as much resistance as if it were passed longitudinally through the 

 fibers of the muscle. In the longitudinal direction the resistance of the muscle 

 is two and a half million times greater than that of mercury. Tetanus and cadaveric 

 rigidity diminish the resistance in the muscles. If the conduction-resistance be 

 tested with alternating currents much lower figures are obtained than if the 

 constant current be employed, because the occurrence of polarization, especially 

 internal polarization, can largely be omitted from consideration. 



The resistance of the body varies between 260 and 1,250,000 ohms. It is 

 high in cases of hysteria and melancholia, and low in cases of tetanus and ex- 

 ophthalmic goiter. Alt and Schmidt make the following statements as to the 

 degree of conduction-resistance in various tissues: Nerve 0.17, muscle i, blood i, 

 skin 1.25. brain 1.57, tendon 3.25, fat 3.92, muscle-sheath 4.41, bone 14.1. 



From Ohm's law two laws of great importance in electro-physiology may be 



