338 RESPIRATION. [CH. 



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 complicated 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 object of respiration being the interchange of gases in the 

 lungs, it is necessary that the atmospheric air should pass into 

 them, and that the changed air should be expelled from them. 

 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 

 point opposite the third dorsal vertebra, where it divides into the 

 two bronchi, one for each lung (fig. 308). It measures, on an 

 average, four or four and a half inches in length and from three-^ 

 quarters of an inch to an inch in diameter, and is essentially a 

 tube of fibro-elastic membrane, within the layers of which are 

 imbedded a series of cartilaginous rings, from sixteen to twenty in 

 number. These rings extend only around the front and sides 

 of the trachea .(about two-thirds of its circumference), and are 

 deficient behind ; the interval between their posterior extremities 

 is bridged over by a continuation of the fibrous membrane in 

 which they are enclosed (tig. 309). The cartilages of the trachea 

 and bronchial tubes are of the hyaline variety. 



Immediately within this tube, at the back, is a layer of 

 unstriped muscular fibres, which extends, transversely, between 

 the ends of the cartilaginous rings to which they are attached, 

 and opposite the intervals between them also ; their function 



