240 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



but also warm the air. It has been calculated by- 

 Bert that, in a bird weighing 1,600 grammes, and 

 having a volume equal to 1,230 cubic centimetres, or, 



in other words, a specific gravity of 1 *3 ( ), 200 



\1230 ' 



cubic centimetres of air can be introduced ; as these 

 200 cubic centimetres weigh -22 of a gramme, it is clear 

 that the specific gravity of the animal will be reduced 



- 10 /1600 + 0-22 1600-22V,, 

 10 1-1S (l230 + 200 OT 1T>* 



must often take into their lungs air at a very low 

 temperature, but with this cold air there is com- 

 mingled that which returns to the lungs from the 

 warm viscera, and by this means the temperature of 

 the respired air is raised ; yet again, such cold air, or, 

 still more, the air of a desert, is often of great dryness, 

 while that which returns from the air sacs has been 

 moistened by the walls of these outgrowths. 



The maximum of complexity is attained by the 

 lungs of the Mammalia, which, occupying a com- 

 paratively smaller space in the body, have nevertheless 

 a much larger area of respiratory surface ; externally 

 the lungs are frequently subdivided into two or more 

 lobes. It has been calculated by Aeby that the 

 human lung contains from three to four millions of 

 pulmonary vesicles, and that in man the respiratory 

 mucous membrane has, in a period of repose, a 

 superficial area of 79 '28 square metres, which 

 can be extended to more than half as much again, 

 or 129 '8 4 square metres; the extent of respiratory 

 surface in the female is rather less than that of 

 the male. 



The air is brought into the lungs from the nasal 

 passages by the trachea, and that tube, as we know, 

 divides into two bronchi, which, in the Amniota, 



* I have corrected what appears to be an error in Bert's calcu- 

 lation. 



