Electroencephalogram—Brain, Nervous Tissue

Richard Caton registered currents from the brain (early form of electroencephalogram [EEG]) in 1875. The problem of registering the activity of the heart was more difficult because the galvanometers of the time were not sufficiently quick. 

The ECG method spread worldwide much more quickly than the other two. But the world record had the spread of X-ray imaging technology. Within months, the first images were taken all over United States and Europe. Medical technology spread much faster than today. 

Hermann von Helmholtz was a student of Du Bois-Reymond. He measured the conduction velocity of a nerve cell axon around 1850. He formulated the very basic theorems of superposition and reciprocity, and also some very important laws of 

Based upon Faraday s work, James Clerk Maxwell published his famous equations in 1873. He more specifically calculated the resistance of a homogeneous suspension of uniform spheres (also coated, two-phase spheres) as a function of the volume concentration of the spheres. This is the basic mathematical model for cell suspensions and tissues still used today. However, it was not Maxwell himself who in 1873 formulated the four equations we know today as Maxwell s equations. Maxwell used the concept of quaternions, and the equations did not have the modern form of compactness he used 20 equations and 20 variables. It was Oliver Heaviside (1850—1925) who first expressed them in the form we know today. It was also Heaviside who coined the terms impedance (1886), conductance (1885), permeability (1885), admittance (1887), and permittance, which later became susceptance. 

Around 1900, it was well-known that large high-frequency currents of more than 1 A could pass through the human body with only heat sensation (Arsine d Arsonval, 1893), but that small low-frequency currents excited the nerves without heat effects. 

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