163 HANDBOOK OF PHYSIOLOGY. 



the " pulmonary " heart; while the left side is "systemic" in function. 

 In many of the lower animals, however, no such distinction can be 

 drawn. Thus, in Fish the heart propels the blood to the respiratory or- 

 gans (gills); but there is no contractile sac corresponding to the left 

 side of the heart, to propel the blood directly into the systemic vessels. 



It may be well to state here that the lungs 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 m inner, of those combustion pro- 

 cesses of which the production of carbonic acid is 1 the final result. 

 These occur in all parts of the body more in one part, less in another: 

 chiefly in the substance of the tissues. 



The Respiratory Passages and Tissues. 



The object of respiration being the interchange of gases in the lungs, 

 it is necessary that the atmospheric air shall pass into them and be ex- 

 pelled 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, thence it passes through the larynx 

 into the trachea or windpipe, which about the middle of the chest di- 

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



The Larynx is the upper part of the passage which leads exclusively 

 to the lung: it is formed by the thyroid, cricoid, and arytenoid cartilages 

 (Fig. 144), and contains the vocal cords, by the vibration of which the 

 voice is chiefly produced. These vocal cords are ligamentous bands at- 

 tached to certain cartilages capable of movement by muscles. By their 

 approximation the cords can entirely close the entrance into the larynx; 

 but under ordinary conditions, the entrance of the larynx is formed by 

 a more or less triangular chink between them, called the rinia glottidis. 

 Projecting at an acute angle between the base of the tongue and the 

 larynx to which it is attached, is a leaf-shaped cartilage, with its larger 

 extremity free, called the epiglottis (Fig. 144, e). The whole of the 

 larynx is lined by mucous membrane, which, however, is extremely thin 

 over the vocal cords. At its lower extremity the larynx joins the 

 trachea. 1 With the exception of the epiglottis and the so-called corni- 

 cula laryngis, the cartilages of the larynx are of the hyaline variety. 



Structure of the Epiglottis. The supporting cartilage of the epi- 

 glottis is composed of yellow elastic cartilage, inclosed in a fibrous 

 sheath (perichondrium), and covered on both sides with mucous mem- 

 brane. The anterior surface, which looks towards the back of the 

 tongue, is covered with mucous membrane, the basis of which is fibrous 



1 A detailed account of the structure and function of the Larynx will be found in 

 Chapter XVI. 



