CHAPTER IX. 

 THE RESPIRATORY SYSTEM. 



SECTION I. 



RESPIRATION consists in the transference of oxygen from the atmo- 

 spheric air to the tissues of the body, and of carbonic acid from the 

 tissues to the outer air. In man and most vertebrates, the oxygen and 

 carbonic acid are carried to and from the tissues respectively by the 

 blood, which, on the one hand, receives oxygen in the lungs and gives 

 it up to the tissues, and, on the other hand, receives carbonic acid from 

 the tissues and gives it up in the lungs. In fishes and many in- 

 vertebrates, the lungs are replaced by gills. The transference of oxygen 

 from the atmosphere into the blood and of carbonic acid from the 

 blood into the atmosphere is called external respiration, the interchange 

 of gases between the blood and the tissues being termed internal or 

 tissue respiration. 



THE STRUCTURE OF THE AIR PASSAGES AND LUNGS. 



The respiratory system consists of the lungs and the air passages 

 leading to them, namely the mouth and lower half of the nasal cavity, 

 the upper part of the pharynx, the larynx, trachea, and bronchi. The 

 trachea is a wide tube about 4^- inches in length in man, and is lined 

 by stratified epithelium, the inner layer being ciliated. The epithelium 

 rests upon a thick basement membrane, beneath which is a layer of 

 elastic fibres running longitudinally ; external to this membrane is 

 areolar tissue in which lie many small glands, which secrete mucin. 

 The trachea is strengthened by C-shaped hoops of cartilage, and on its 

 posterior wall is a layer of unstriped muscle, the fibres of which run 

 transversely. The cartilaginous rings keep the lumen of the trachea 

 patent, and prevent its occlusion by external pressure. The fluid 

 formed by the small mucous glands moistens the inner surface of the 

 trachea, and serves also to catch bacteria or particles of dust, which 



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