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 the nervous system; vital spirits governing most of the other functions; and finally, natural spirits regulating the liver and susceptible of incorporation in the blood. In the sixteenth century, in the time of Paracelsus, Galen's spirits became Olympic spirits. They still presided over the functional activity of the organs, the liver, heart, and brain, but they also existed in all the bodies of nature.

Van Helmont.—Finally, the theory was laid down by Van Helmont, physician, chemist, experimentalist, and philosopher, endowed with a rare and penetrating intellect. Here we find many profound truths combined with fantastic dreams. Refusing to admit the direct action of an immaterial agent, such as the soul, on inert matter, on the body, he filled up the abyss which separated them by creating a whole hierarchy of immaterial principles which played the part of mediators and executive agents. At the head of this hierarchy was placed the thinking and immortal soul; below was the sensitive and mortal soul, having for its minister the principal archeus, the aura vitalis, a kind of incorporeal agent, which is remarkably like the vital principle, and which had its seat at the orifice of the stomach. Below again were the subordinate agents, the blas, or vulcans placed in each organ, and intelligently directing its mechanism like skillful workmen.

These chimerical ideas are not, however, so far astray as the theory of vital properties. When we see a muscle contract, we say that this phenomenon is due to a vital property—i.e., a property without any analogue in the physical world, namely contractility, In the same way the nerve possesses two vital properties, excitability and conductibility, which Vulpian