CHAPTEE XXIV 



RESPIRATION 



THE respiratory apparatus consists of the lungs and of the air-passages 

 which lead to them. In marine animals the gills fulfil the same 

 functions as the lungs of air-breathing animals. The muscles which 

 move the thorax and the nerves that supply them must also be in- 

 cluded under the general heading Kespiratory System ; and, using 

 this expression in the widest sense, it includes practically all the 

 tissues of the body, since they are all concerned in the using up of 

 oxygen and the production of waste materials, like carbonic acid. 



Essentially a lung or gill is constructed of a thin membrane, one 

 surface of which is exposed to the air or water, as the case may be, 

 while, on the other is a network of blood-vessels the only separation 

 between the blood and aerating medium being the thin wall of the 

 blood-vessels, and the fine membrane on one side of which vessels are 

 distributed. The difference between the simplest and the most com- 

 plicated respiratory membrane is one of degree only. 



The lungs or gills are only the medium for the exchange, on the 

 part of the blood, of carbonic acid for oxygen. They are not the seat, 

 in any special manner, of those combustion-processes of which the 

 production of carbonic acid is the final result. These processes occur 

 in all parts of the body in the substance of the tissues. 



The Respiratory Apparatus. 



The lungs are contained in the chest or thorax, which is a closed 

 cavity having no communication with the outside except by means of 

 the respiratory passages. The air enters these passages through the 

 nostrils or through the mouth, whence it passes through the larynx into 

 the trachea or windpipe, which about the middle of the chest divides 

 into two tubes, bronchi, one to each (right and left) lung. 



The Larynx is the upper part of the passage, and will be described 

 in connection with the voice. 



The Trachea and Bronchi. The trachea extends from the cricoid 

 cartilage, which is on a level with the fifth cervical vertebra, to a 



