90 SCIENCE PRIMERS. [§ vi. 



and changes from impure and venous to 

 pure and arterial. 



39. Through the capillaries of the muscle a stream 

 of blood is ever flowing so long as life lasts and the 

 heart has power to beat ; every instant a fresh supply 

 of bright, pure, arterial blood comes to take the place 

 of that which has become dark, venous, and impure. 

 Without this constant renewal of its blood the muscle 

 would be choked, and its vital flame would flicker and 

 die out. 



In the lungs, the air filling the air-cells would if 

 left to itself soon lose all its oxygen and become 

 loaded with carbonic acid ; and the blood in the 

 capillaries of the lungs would no longer be changed 

 from venous to arterial, but would travel on to the 

 pulmonary vein as dark and impure as in the pulmo- 

 nary artery. Just as the blood in the muscle 

 must be constantly renewed, so must the air 

 in the lungs be continually changed. 



How is this renewal of the air in the lungs brought 

 about? 



In the dead rabbit you saw the lungs, shrunk, 

 collapsed, emptied of much of their air, and lying 

 almost hidden at the back of the chest (Fig. i, G.G,) 

 The cavity of thqf chest seemed to be a great 

 empty space, hardly half filled by the lungs and 

 heart. Bat this is quite an unnatural condition of the 

 lungs. Take another rabbit, and before you touch 

 the chest at all, open the abdomen and remove all its 

 contents — stomach, liver, intestines, &c. You will then 

 get a capital view of the diaphragm, which as you 

 already know forms a complete partition between the 

 chest and the belly. You will notice that it is arched 



