MULTICELLULAR ANIMALS 171 



The vocal cords are in reality membranes continuous with the 

 wall of the larynx that stand over and partially obstruct the passage. 

 In ordinary breathing they are relaxed and permit passage of air 

 without any vibration whatever. When the voice is produced these 

 membranes are tautened and the air is forced against them with 

 some pressure so that they vibrate, the frequency of vibration being 

 regulated by the degree of tautness. The cavities of the thorax, the 

 column of air in the trachea, and the cavities of the nose and 

 sinuses of the bones of the face constitute resonating cavities. Mo- 

 tions of the tongue, jaws, lips, and soft palate modify the sounds 

 produced. In general, the vocal cords vibrate only while air is being 

 expired, but in some mammals sounds are produced on inspiration 

 also, for example in the bray of the donkey. 



The TRACHEA (Fig. 115) leads from the larynx, and is composed of 

 rings of cartilage, so that it is always open. It branches into two 

 BRONCHii, one leading toward each lung. These further subdivide 

 into BRONCHIOLES, which divide repeatedly, becoming smaller and 

 smaller; finally ending in small air sacs, termed alveoli. Normally 

 the alveoli always contain some residual air. The air having rushed 

 in by the increasing of the capacity of the pleural cavities by the 

 lowering of the diaphragm and the raising of the ribs, the inter- 

 change of gases takes place in the alveoli with great rapidity, and the 

 air is expelled by the contraction of the muscles of the body wall and 

 diaphragm. The walls of the trachea and bronchii are lined with 

 cells that are ciliated. These by constant and correlated beating 

 maintain an outward flow of viscous liquid secreted by some of the 

 cells lining the passages. This current serves to remove dust and 

 bacteria that have entered with the air. 



The exchange of gases in the alveoli is considered as a simple 

 diffusion through the membranous walls. It is estimated that the 

 total area of the alveoli in the human lungs is in the vicinity of go 

 square yards. The diffusion of gases in the lungs is not respiration; 

 it is merely the introducing of oxygen into, and the passage of 



