LUNGS 283 



of the body; that they play a part in the reduction of the bodily tem- 

 perature in correlation with the absence of sweat glands, and that, 

 where they extend between muscles, they reduce the friction. More 

 plausible is the view that, by the motion of the parts by which they 

 are surrounded, they aid in the inspiration and expiration of the 

 air, especially during flight, thus allowing the thoracic framework to 

 remain rigid as an attachment of the muscles, and at the same time 

 permitting the air to pass twice over the respiratory surfaces of 

 the lungs. 



MAMMALS. — The general structure of the mammalian lung is 

 outlined above (p. 275). Each lung is entirely enclosed by the 

 pleural membrane, and the pleural cavity in which it lies is entirely 

 cut off from the rest of the ccelom by the diaphragm, which is usually 

 transverse to the main axis of the body, but is oblique in the whales. 

 The external shape of the lung is largely due to the position in the 

 pleural cavity, where it has to fit on either side of the pericardium, 

 while the presence of the diaphragm causes the posterior surface to be 

 truncate. In some mammals the lungs are simple and without 

 division into lobules (whales, elephant, odd-toed ungulates, Hyrax) 

 and in the monotremes only the right is divided. Elsewhere both 

 lungs are lobed, but the lobes (varying in number from two to six) 

 are more numerous on the right side. 



Internally there is a main bronchus from which dorsal and ventral 

 secondary bronchi arise,the latter dividing in a dichotomous manner. 

 In most mammals the bronchi are strengthened by cartilages which 

 form rings in the larger, scattered pieces in the smaller trunks. Fre- 

 quently one or both anterior (apical) bronchi lie in front of or above 

 the pulmonary artery and these are called eparterial bronchi, the 

 others being hyparterial, but the distinction is of little morphological 

 importance. 



In one of the whales (Ponioporia) the eparterial bronchus of one side leaves 

 the main trunk close to the larynx and outside the lung, so that here there are 

 apparently three main bronchi. It is estimated that there are over 400,000,000 

 alveoli in the human lung. 



The phylogenetic history of the lungs is uncertain, one view being 

 that they have developed from the air bladder of fishes, the other is 

 that they are modified gill pouches, which, instead of growing later- 

 ally and opening to the exterior, have extended caudally and have en- 

 croached upon the coelom. Favoring the first of these views are the 

 double condition of the bladder in some ganoids, and especially in 



