THE SOURCES OF ENERGY 67 



ception of an automatic, mechanical, animal body, and no such 

 momentous contribution to a working hypothesis of life had 

 before then, or has since, been made. 



The Respiratory Interchange. — Thus food materials are taken 

 into the alimentary canal, digested, and transferred to the blood- 

 stream in such a form that they can be assimilated by the tissues. 

 They are then distributed in the blood-stream to all parts of the 

 body. 



But simultaneously there is an interchange of something 

 between the blood itself and the air that is inspired by the lungs. 

 Here we mention, for the first time, the arterial and venous 

 kinds of blood. That which issues from a cut vein, and in 

 general from a slight wound, is blood that is rather dark red in 

 colour, and which oozes out from the cut vessels, whereas that 



Artery 



ir tube in 

 Tconnection v^lth 



bronchvs 



Air Vesicle "^ 



Fig. 16. — An Alveolus, ob Air Sac, from the Lungs, with its 

 Bloodvessels. Highly Magnified. 



which comes from an artery is brighter red in colour and it flows 

 out in pulsations. Examining these bloods, physiologists have 

 found that that flowing in the arteries contains more oxygen and 

 less carbonic acid gas than that flowing in the veins. Examining 

 the air that is inspired, they find that this contains the usual 

 21 per cent, of oxygen and about ^V P^r cent, of carbonic acid, 

 while the expired air contains only about 16 per cent, of oxygen 

 and about 4 per cent, of carbonic acid. 



Both the composition of the blood and that of the air are 

 changed in the lungs. Now look at the structure of the latter: 

 each consists essentially of an immense number of air vesicles 

 surrounded by capillary bloodvessels. 



This is a scheme of the essential structure of the lung. The 

 trachea divides to form the two bronchi, and the latter divide 

 and subdivide again and again until they terminate in multitudes 

 of sacs, or vesicles, one of which the greatly simplified figure 



