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THE AMERICAN NATURALIST [Vol. XLII 



tific, together with other, learning, and well into the 

 seventeenth century, the doctrine of the pneuma, or 

 spirits, reigned supreme. This doctrine was often ex- 

 pressed in vague, uncertain terms, and in the hands of 

 the Stoic philosophers, the Pneumatic physicians, the 

 scientific men of Alexandria, Galen and minor writers, 

 it was variously portrayed. The pneuma was believed 

 to be an extremely subtile material agent entering the 

 body with the breath, and was spoken of as "very subtle 

 air," "very lively and pure flame," "fluid," "of the 

 nature of light," "vapor," "something analogous to the 

 spirits of wine," and so on. Each vital action was a 

 manifestation of its activity. In the heart dwelt the vital 

 spirits ; in the brain the animal spirits. They flowed and 

 ebbed through the veins and arteries, they coursed along 

 the nerves, they permeated the tissues, and they bubbled 

 and effervesced. Through them the body felt and moved, 

 was nourished and warmed, grew and reproduced. It 

 was a genial, comforting belief, nothing was more plaus- 

 ible ; in its light vital actions seemed simple enough ; and 

 so for two thousand years the spirits danced merrily 

 along. 



But there were hard-headed thinkers in the seven- 

 teenth century. We may even imagine that the skilful 

 experimenter, Harvey, the discoverer of the circulation of 

 the blood, had his doubts. It is true that Descartes made 

 free use of the spirits in his clever portrayal of the work- 

 ing of the organic machine, but there were others, be- 

 lievers in the machine, who contended that its Deus was 

 not the pneuma. The spirit of mechanism was in the 

 air. With the beginning of a rational physics, stimulated 

 largely by the discoveries of Galileo and Newton, and a 

 rational chemistry, freed from alchemy, there arose those 



cists and the iatro-clitMnists!7ed!^^^ 



being through the spectacles of the physici< the other 

 through those of the chemist; to the one vital actions 



