INSPIRATION. 91 



CHAPTEE VII. 



RESPIRATION. 



1. Why we Breathe. In the preceding chapter we 

 traced the circulation of the blood, and noticed that, in pass- 

 ing through the capillaries, it gives to the cells of each tissue 

 the food and fuel they require, while, at the same time, it 

 receives from the tissues certain elements of decay. The 

 arterial blood, freighted with oxygen, is changed into venous 

 blood, laden with carbonic acid and other products of chemi 

 cal change. In the round of circulation, the venous blood is 

 carried to the lungs. To preserve the system in health, these 

 impurities must be got rid of, and it is the office of the 

 respiratory system to discharge this important duty, and to 

 give back to the blood a fresh supply of oxygen from the 

 air. The act of breathing is necessary for life. It might 

 even be suspended for some minutes in rare instances, but 

 the demand for air is imperative, and if not given, life is 

 forfeited. 



2. The Organs of Respiration. These consist of the 



lungs, the air passages and the pulmonary vessels, with their 

 branches. They are situated in the chest, and with the heart 

 and large blood-vessels, fill the thoracic cavity. 



3. The Lungs. There are two lungs, the right and the 

 left. They are separated by the heart and large blood-vessels. 

 Each lung is conical in shape, the apex fitting into the upper 

 pointed cavity of the chest, immediately under the collar- 

 bone, the base resting on the diaphragm. The outer surface 

 is rounded, to fit the curve of the ribs. The inner surface is 



