338 



PHYSIOLOGY. 



muscles of respiration, when called into play, make the thorax act 

 as a bellows, forcibly causing ingress and egress of air. 



The Air-passages. The very first portion of the respiratory pas- 

 sageway, the nose, is the organ of the special sense of smell and will 

 be treated in detail when that subject is discussed; the anatomy of 

 the pharynx has been previously noted when the alimentary canal was 

 under attention. The larynx is placed at the upper part of the pas- 

 sage, being a dilatation of the trachea. It is the cartilaginous box 

 which contains the structures concerned in the production of voice. 

 It will be described later in connection with that function. 



Fig. 111. Human Respiratory Apparatus. (DuVAL. ) 

 It shows the branching of the bronchia in the interior of the lungs. 



The Trachea and Bronchi. The trachea, or windpipe, is a com- 

 bined membranous and cartilaginous cylindrical tube, flattened pos- 

 teriorly. Commencing opposite the fifth cervical vertebra, it ter- 

 minates by dividing into two bronchi opposite the third dorsal ver- 

 tebra. Its length is about four inches, its breadth (less in the female 

 than in the male), three-fourths of an inch. The bronchi diverge 

 from the trachea to the lungs behind the great blood-vessels running 



