RESPIRATION IN BIRDS AND MAMMALS. 375 



placed together in a limited quantity of air, which gradually becomes 

 vitiated by their respiration, the Bird will die first, the Mammal next, 

 and the Reptile last. Or if the Bird be placed alone in a limited 

 quantity of air, and be left until the atmosphere is so vitiated as to be 

 no longer capable of supporting its life, a Mammal will still live for a 

 time in that atmosphere ; and when it is no longer fit to sustain the 

 life of the Mammal, the Reptile may still breathe it without injury for 

 a considerable period. There is strong reason to believe, indeed, that, 

 in former epochs of the Earth's history, when the Reptile class was 

 predominant, supplying the place of Mammals on land, and of Birds in 

 the air, the atmosphere was so highly charged with carbonic acid, as 

 not to be capable of sustaining the life of the higher air-breathing Ver- 

 tebrata. 



3. Mechanism of Respiration in Mammalia and in Man. 



675. It is in the class of Mammalia that we find the Respiratory 

 apparatus presenting its highest degree of concentration ; and the 

 arrangements for its action the most complete. The Lungs are divided 

 into cavities of extreme minuteness, so that the extent of surface which 

 they expose is enormously increased. These cavities, or air-cells, are 

 all connected with the trachea by means of the bronchial tubes and 

 their minute subdivisions ; but, on account of the minuteness of these 

 passages, a considerable force would be required to inflate the air-cells 

 with air, if their distension were to be accomplished by the propulsion 

 of air through the trachea, as we have seen to be the normal mode of 

 inspiration in Reptiles. Moreover, if the air were introduced in this 

 manner, the air-cells would be the last portions of the pulmonary struc- 

 ture that would be distended by it, as well as the first to be emptied 

 when the air is forced out again by external pressure. The mechanism 

 of Respiration in Mammalia, however, is so arranged, that the air is 

 most effectually drawn into the lungs, instead of being forced into 

 them ; and the distension of the air-cells is far more complete than it 

 could be rendered in the latter method, besides being accomplished in a 

 much shorter time. 



676. The general principle of the operation is this. The lungs are 

 suspended in a cavity that is completely closed, being bounded above 

 and around by the bony framework of the thorax, the interspaces of 

 which are filled up by muscles and membranes, and being entirely cut 

 off from the abdomen below by the diaphragm. Under ordinary cir- 

 cumstances, the lungs completely fill the cavity ; their external surface, 

 covered by the pleura, being everywhere in contact with the pleural 

 lining of the thorax. But the capacity of the thoracic cavity is sus- 

 ceptible of being greatly altered by the movements of the ribs, and by 

 the actions of the diaphragm and abdominal muscles, as will presently 

 be explained in more detail. When it is diminished, the lungs are 

 compressed, and a portion of the air contained in them is expelled 

 through the trachea. On the other hand, when it is increased, the 

 elasticity of the air within the lungs causes them immediately to dilate, 

 so as to fill the vacuum that would otherwise exist in the thoracic 



