568 PHYSIOLOGY OF RESPIRATION. 



functions of motion and sensation. Galen (131 A. D.) demonstrated 

 that the arteries as well as the veins contain blood, but still believed 

 that the chief function of the respiratory movements is to furnish 

 pneuma or vital spirits to the heart. This great physiologist noticed 

 also that the air is necessary for combustion as it is for life, and 

 stated his belief that the explanation of one of these acts would 

 be also an explanation of the other. This thought seems to have 

 been accepted by all the physiologists of subsequent times, but it 

 required over sixteen hundred years of investigation before a satis- 

 factory solution was reached. Galen recognized, moreover, that 

 not only does the blood take something of essential importance from 

 the air, namely, vital spirits, but it also gives off something to the 

 air that is injurious to the body, a something which he compared to 

 the smoke of combustion and designated as the " fuliginous vapor. " 

 If we substitute oxygen for vital spirits and carbon dioxid for 

 fuliginous vapor we realize that the essential problem of respiration 

 was already clearly formulated, but could not make further advance 

 until chemical knowledge was more fully developed. Such is the 

 case with some of our physiological problems to-day. Galen also 

 explained satisfactorily the respiratory movements, the action of the 

 muscles of inspiration and expiration, thus destroying the older 

 erroneous theories that the expansion and contraction of the lungs 

 are due to processes of heating and cooling. 



Galen's physiology held undisputed sway until the seventeenth 

 century. At that time there arose a school of physiologists, the 

 iatromechanists, who proposed to explain all vital phenomena upon 

 known mechanical principles, the laws of physics and chemistry. 

 For the mystical view of vital spirits they proposed to substitute a 

 more rational and concrete theory. The blood in the lungs becomes 

 red simply because it is minutely subdivided and shaken, just as a 

 tube of blood becomes red when violently agitated. Thus an effort 

 to be more scientific, to use the exact knowledge of physics, led to 

 the adoption of views which we now know were far more erroneous 

 than the ancient and intrinsically correct conception that the blood 

 receives something from the air in the lungs. 



In the seventeenth century, however, began those discoveries 

 in chemistry and physiology which eventually led to our present 

 knowledge. Van Helmont (1577-1644) discovered that in the 

 burning of charcoal, the fermentation of wine, and the action of 

 vinegar on chalk a special gas is produced which he called gas 

 sylvestre and which we call carbon dioxid. Robert Boyle (1627- 

 1691) published a most interesting series of experiments made with 

 the aid of the recently discovered air-pump which demonstrated the 

 correctness of the view held by Galen that the air contains some- 

 thing necessary for life and for combustion. He showed, moreover, 



