36 



ZOOLOGY. 



where they would press on the oesophagus. In the larynx 

 the cartilages are more complicated. 



4. Structure of Lungs. On entering the thorax the 

 trachea divides into two tubes the right and left bronchi. 

 These divide again the right into four, the left into two, 



and on each of these six 

 bronchi there is set one of 

 the lobes of the lungs (fig. 5). 

 If we could follow out each 

 l>ronchus as it disappears into 

 the lung-lobe, we should find 

 it branching again and again 

 into smaller and smaller 

 branches, the bronchioles. 

 Each final twig of this tree 

 of tubes ends in a minute 

 sac an air-sac or alveolus 

 in whose thin walls lie 

 Here it is that blood and air, 

 separated by only a thin membrane, exchange oxygen and 

 carbon dioxide. The whole lung-lobe, solid as it at first 

 looks, is little more than a mass of these air-sacs and tubes 

 as its extreme lightness and sponginess testify. 



5. Mechanism of Inspiration. To maintain a constant 

 supply of oxygen to the blood the air must be continually 

 pumped in and out of the lungs. The pumping in is called 

 inspiration ; the pumping out, expiration. In the rabbit 

 this is effected in the following way. The walls of the 

 thorax contain a series of ribs, and between these ribs are 

 two series of muscles, the external and internal intercostal 

 muscles, so arranged that when the former act,* they pull 

 all the ribs forward (raise the ribs, in the case of man), 

 while the latter have the opposite action. In inspiration the 

 ribs are pulled forward : at the same time the muscular dia- 

 phragm (chap, i., 9) pulls backwards, so that the convexity 

 it presents to the thorax (fig. 1) is diminished. Both these 

 changes enlarge the capacity of the thorax and so tend to 



* The ^nature of muscular action is explained in chapter viii, 



Fig. 5. LUNGS OF KABBIT. 



the pulmonary capillaries. 



