THE TRACHEA AND LUNGS 145 



QUESTIONS ON LESSON XXIV. 



1. Describe the general form, size, position, and structure of the trachea. 



2. How does the structure of the trachea differ from that of the oesophagus ? 



3. Describe the general form and structure of the larynx. 



4. How do the smallest branches of the trachea terminate in the lungs ? 



5. What provision is made for the discharge of any foreign matter that may 



find its way into the air-passages? 



6. Describe the general form and position of the lungs. How does the right 



lung differ from the left ? 



7. Describe the manner in which the blood circulates in the lungs. Name 



the vessels concerned in this circulation ? 



8. Where does arterial blood become -venous, and where does venous blood 



become arterial''. 



9. How does the blood which leaves the lungs differ from that which enters 



them? 



IO. How much air enters and leaves the lungs at each breath ? To what ex- 

 tent may this quantity vary ? What is the greatest, and what is the 

 least quantity of air which the lungs may contain during life ? 



LESSON XXV. 



WHY WE BREATHE VENTILATION. 



IN Lesson XV. we learnt that all living tissues were constantly 

 wasting away, and that not only was it necessary that they should 

 be repaired, but also that the waste substances should be removed 

 from the system. 



Now, the lungs are included among those organs which remove 

 waste matter, but we have to learn that they are at the same time 

 sources of gain as well as of loss to the system, or at least to the 

 blood. 



We have noticed that arterial blood becomes venous in the 

 capillaries of the tissues of most parts. This is due to the fact 

 that some of its oxygen has united with carbon and hydrogen of the 

 waste matters, forming carbonic acid gas and water. The oxygen 

 supply was obtained from the air-cells of the lungs, and was con- 

 veyed to all parts by the blood stream. 



Life could not be maintained for even a short time without 

 this chemical combination, for not only is it a great source of heat, 

 by which the high temperature (99 F.) of the body is maintained, 

 but is also a means by which waste organic matter is converted 

 into simpler compounds (carbonic acid gas, wafer, and urea) pre- 

 vious to removal. 



Thus the lungs are a source of gain to the blood ; and the 

 oxygen imported by these organs is utilised in converting waste 

 or dead animal matters into substances which can readily be taken 



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