224 SPEEDS OF SOME PROCESSES IN BIOLOGICAL SYSTEMS 



The electrodes of the electrocardiograph (EKG) and electroencephalo- 

 graph (EEG) are placed on the outside of such a volume conductor, the 

 body, and measure potential differences between points in outside paths. If 

 the concentrations of salts remain constant throughout the body, as they 

 should in the steady-state, then any variations in the voltage measured 

 should reflect variations in the internal currents resulting from variations in 

 source voltage, E. 



In biological systems the source of the potential difference between differ- 

 ent places or spots is invariably a concentration difference, whether uni- 

 ionic or bi-ionic. Concentration differences occur for two reasons: (1) ion 

 selectivity of membranes, and (2) continuous exchange with the medium 

 through which the distribution systems (blood and lymph systems) pass. 

 Membrane potentials cancel out over the whole system, because the im- 

 portant ion selectors are cell walls, which completely enclose and isolate a 

 volume. In the absence of disturbances then, concentration difference is 

 the source of the bioelectric potentials. 



However, two major disturbances exist, both of which "irritate" the mem- 

 branes of the cell wall, cause them to become permeable, and thereby reduce 

 the selectivity and permit mixing of otherwise separated salts. One is the 

 mechanical pressure variations transmitted through the blood stream by the 

 heart; the other is the electrical polarizing action of nerve. The former causes 

 a concentration change by the application of a mechanical force, the latter 

 by electrical interference with the membrane potentials of cells. Potential 

 variations with time, between electrodes on the skull, above different lobes 

 of the brain, give a precise record of the electrical action within the meas- 

 ured region; and electrodes placed on the torso and leg at spots where a 

 major artery runs close to the surface, give a reliable record of the pumping 

 action of the heart. Since any mechanical stimulus will cause momentary 

 irritation (and therefore potential variations), the measurements are always 

 made under controlled conditions when the electrical "noise" generated by 

 the involuntary muscles of the organs (and always present) is at a minimum. 



ON HEAT CONDUCTION; 98.6° F: A CONSTANT? 



Heat Production 



The human body has a heat capacity, as does any other, measured as the 

 heat in calories required to raise 1 g 1°C. Also, the ambient (surrounding 

 temperature may vary widely — for example, from 95° F (35° C) down to 

 — 20° F ( — 30° C). This lower value is 67 Centigrade degrees, or 1 19 Fahren- 

 heit degrees, below body temperature, and yet the body is able to maintain 

 within a small fraction of a degree the normal value of 37° C. Admittedly, 

 insulation-aids such as skin, clothing, and hair play a large part; and the 



