CHAPTER XXII. 



THE RESPIRATION (Cont'd). 



The External Respiration. 



Anatomical Considerations. The constant call of the tissues 

 for oxygen and the formation of the waste gas, carbon dioxide, 

 demands a mechanism by which the blood can continually renew 

 its supply of oxygen and excrete its excess of carbon dioxide. 

 This exchange, as we have seen, is effected in the lungs, which 

 are built up in the following way : 



The nasal and oral cavities lead to the pharynx, from which 

 open two tubes: one posterior, the ossophagus, going to the ali- 

 mentary tract, and the other, anterior the trachea, going to the 



Fig. 28. Diagram of structure of lungs showing larynx, bronchi, bron- 

 chioles and alveoli. 



lungs (Fig. 28). At the beginning of the trachea is placed the 

 larynx, or the voice box, the opening of which is guarded by a 

 flap of tissue, the epiglottis. Within the larynx are the vocal 

 cords. The trachea, or windpipe, is a relatively large tube, about 

 four and one-half inches long, which, after its entrance into the 

 thorax, divides into two tubes, the bronchi, each of which subdi- 

 vides again and again, the branches gradually growing smaller 

 until they are mere twigs, and are known as bronchioles, or small 



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