32 WONDERS OF ORGANIC LIFE. 



to everything, are the result. Again, let the 

 insensible perspiration of the skin be checked, 

 or suspended, and how soon the lungs and 

 internal organs generally begin to suffer, and 

 congestion or inflammation takes place. Let 

 the excretion of the mucous membranes become 

 greatly diminished, and manifold are the dis- 

 eases which immediately supervene. 



It is, however, to that purifying process in 

 the lungs by which dark carbonized venous 

 blood is converted into bright red arterial blood, 

 that w r e would here more especially advert. 

 This process is respiration. In animals with 

 lungs, respiration consists of two acts, inspira- 

 tion (performed in tortoises and frogs by a sort 

 of deglutition) and expiration. Without enter- 

 ing minutely into the structure of the lungs, we 

 may state that the bronchial tubes, or branches 

 of the windpipe, pass gradually into a collection 

 of minute vesicles, consisting of exquisitely fine 

 membranes, over which the capillary branches 

 of the pulmonary artery ramify ; and that these 

 membranes and capillary branches are perme- 

 able by air, which is brought into immediate 

 contact with the blood. The pulmonary artery 

 brings dark venous blood from the right side 

 of the heart (the right ventricle) to the lungs ; 

 the pulmonary veins convey the purified blood 

 from the lungs to the left side of the heart, (the 

 left auricle, whence it passes into the left ven- 

 tricle,) from which it is sent through all the 

 arteries of the system. 



At each act of inspiration, as the air rushes 



