CHAPTER XIII. 



THE RESPIRATORY APPARATUS: LARYNX; TRACHEA; LUNGS. 

 RESPIRATION; EFFECTS OF RESPIRATION UPON THE AIR 

 WITHIN THE LUNGS, UPON THE AIR OUTSIDE THE BODY, 

 UPON THE BLOOD; MODIFIED RESPIRATORY MOVEMENTS. 



The respiratory apparatus. — Respiration is the main process 

 by means of which the body is supplied with oxygen and re- 

 lieved of carbon dioxide. 



A respiratory apparatus consists essentially of a moist and per- 

 meable membrane, with blood-vessels containing carbon dioxide 

 on one side, and air or fluid containing oxygen on the other. 

 In most aquatic animals, the respiratory organs are external in 

 the form of gills ; in terrestrial or air-breathing animals, the 

 respirator}^ organs are situated internally under the form of 

 lungs, and are placed in communication with the external air 

 by a tube or windpipe. 



In man, the respiratory apparatus may be conveniently di- 

 vided into the larynx, trachea, and lungs. 



The larynx. — The larynx is situated between the base of the 

 tongue and the top of the trachea, in the upper and front part 

 of the neck. Above and behind lies the pharynx, which opens 

 into the oesophagus or gullet, and on either side of it lie the great 

 vessels of the neck. 



The larynx is made up of nine pieces of cartilage, united 

 together by ligaments, and moved by numerous muscles. It is 

 lined throughout by mucous membrane,^ which is continuous 

 abovf with that lining the pharynx, and below with that lining 

 the trachea. In form, the larynx is narrow and rounded below 



^ Mucous membranes resemble the skin in structure, and may be said to 

 form an internal skin for the cavities of the body which open exteriorly. They 

 always have a basis of connective tissue, are lined with epithelium, and secrete 

 a sticky substance called mucus. For a further description, see page 166. 



151 



