WHY WE BREATHE VENTILATION 147 



When an animal is strangled, choked, or drowned, the same 

 disease (asphyxia) ensues. In these cases no oxygen can enter 

 the blood, while the carbonic acid gas is fast accumulating by the 

 oxidation of the tissues, and soon saturates the blood, all of which 

 rapidly becomes venous. The occurrence of death in a few 

 minutes show? the importance of oxygen gas in the renewing of 

 the blood. 



We have now learnt the necessity of ventilating our rooms 

 that is, of allowing for the removal of the expired air, and for the 

 access of pure air from the outside atmosphere to take its place. 



QUESTIONS ON LESSON XXV. 



i. What changes take place in blood as it passes (i) through the capillaries 

 of the lungs, and (2) through the capillaries of the muscles 1 



z. What is the temperature of the blood 1 How is this temperature 

 maintained 1 



3. What is the difference between inspired and expired air 1 



4. What is asphyxia 1 How is it caused 1 



5. What is the direct cause of death when an animal is strangled 1 



6. How is it that a person turns black in- the face when he is choked 1 



7. If a number of people were shut up in a close room without any ventilation, 



what changes would the atmosphere undergo 1 What would be the 

 probable condition of the atmosphere (i) when the people began to 

 complain, and (2) when they began to die 1 



8. What is meant by oxidation 1 How do we know that oxidation is always 



going on in the body t 



LESSON XXVI. 



HOW WE BREATHE. 



BEFORE we go on to describe the manner in which the air is 

 caused to enter and leave the lungs, it will be desirable to recall 

 a few particulars concerning the structure of the chest and the 

 lungs. 



The walls of the chest are formed by the back bone behind, 

 the breast bone or sternum in front, the ribs with their intercostal 

 muscles around the sides, and the membranous and muscular 

 diaphragm below. The heads of the ribs form mova&ie joints 

 with the dorsal vertebrae, and the other ends of the ribs are con- 

 nected in front with the sternum by the flexible costal cartilages 

 The lungs fill the greater part of the conical cavity of the thorax 

 all excepting the small portion occupied by the lower trachea, 

 the oesophagus, and the heart with its great vessels. They are 

 enclosed in a double membrane the pleura one layer of which 



L 2 



