RESPIRATORY, OR BREATHING, ORGANS. 403 



windpipe, and that part of the pleura which divides the chest. 

 They are of much the same shape in all larger animals, being 

 exactly suited to the cavities in which they lie. 



The lungs are composed of the branches of the bronchial 

 tubes, the air-cells, and the branches of the arteries and veins 

 of the lungs, all of which are held together by a spongy, whit- 

 ish, cellular substance, called the parenchyma. The capillary 

 vessels of the lungs are ver}^ numerous. Those forming the 

 connection betAveen the branches of the pulmonary arteries 

 and veins are spread out over the air-cells, forming a complete 

 net-work envelope for them. By this arrangement it will be 

 seen that every particle of blood brought to the lungs by the 

 pulmonary artery must pass over the surface of the air-cells, 

 before reaching the pulmonary veins to be carried back to the 

 heart, JN'ow, the walls of the air-cells and capillaries are so 

 thin as to allow the poisonous carbonic acid, which the blood 

 gathers up in its course through the system, to pass out, and 

 the oxygen of the air to pass to the blood and again fit it for 

 sustaining life. 



The change which the blood undergoes in the lungs is 

 absolutely necessary to sustain life. This change is called 

 the arterialization of the blood. It consists in freeing the 

 blood of carbonic acid gas, and charging it with oxygen, and 

 which causes its color to change from dark to bright scar- 

 let. If this change did not take place, life could last but 

 a few seconds. 



The arteries and veins which carry blood to build up 

 and repair the substance of the lungs, pass through them 

 just as those of other parts. 



The iileura is a thin, tough membrane, vrhich lines the 

 inside of the chest over the ribs and diaphragm, and is also 

 spread over the lungs, and a broad fold of it dips down be- 

 tAveen them, forming the partition between the tAvo sides of 

 the chest. The pleura, as it is spread OA^r the ribs, dia- 

 phragm, and lungs, is listened to these parts. Its free sur- 



