PROVISIONS FOR THE FUNCTION. 393 



525. In the warm-blooded Vertebrata, which Fig. 96. 

 have a complete double circulation, namely, 



birds and mammalia, a much larger extent of 

 surface is provided for the aeration of the blood; 

 the whole current of which is transmitted to the 

 lungs, before circulating again through the sys- 

 tem. This increase is provided in birds, partly 

 by the more minute subdivision of the lungs into 

 cells, and partly by the addition of a number of 

 large air-sacs, which are disposed in various parts 

 of the body, and even in the interior of the long 

 bones. Hence it happens, that the amount of Potion of the Lung of a p/^, the 



... I'll- terminal vessels being filled with 



respiration is greater m this class than in any mercury . A , natural 8ize . B) mode . 

 other, although the form of the apparatus is not ra tei y magnified. (After Wagner.) 

 nearly so concentrated as in the mammalia ; nor is 

 the mechanism of the chest so well adapted to a 



constant exchange of the air contained in its cavities ( 48). In mammalia 

 the lungs are proportionally smaller, and the whole respiratory apparatus is 

 restricted to the thorax ; but the minute subdivision of their cavity, and the 

 mechanism by which a continual interchange of air is provided for, render 

 them very efficient for their designed purpose. In regard to the intimate 

 structure of the lungs of man and of the mammalia, it is difficult to speak with 

 confidence. It was maintained by Reissessen, and has been repeated by other 

 anatomists, that the air-cells of the lungs are in reality the globular dilatations 

 of the extremities of the ultimate ramifications of the bronchial tubes, analogous 

 to the milk-cells of the mammary gland (Fig. 161) ; but it has been objected, 

 that they are much more numerous than these ramifications can be supposed 

 to be ; and there seems much reason to believe that every tube leads to a clus- 

 ter of cells, communicating with each other. The recent inquiries of Mr. 

 Addison* have shown that Reissessen's account is true of the foetal lung ; in 

 which the ultimate subdivisions of the bronchial tubes terminate without anas- 

 tomosis in closed extremities. But when an animal has respired, these termi- 

 nations undergo a great change ; for the membrane composing each of them 

 offers but a feeble resistance to the pressure of the air, and is pushed forwards 

 and distended laterally into rounded inflations, forming a series of cells, which 

 are moulded by mutual pressure into various angular forms, and which com- 

 municate freely with one another by large oval apertures. The passages thus 

 formed do not communicate with each other, otherwise than by their connec- 

 tion with the same bronchial tube ; and the blood-vessels lie between the con- 

 tiguous walls of each two of them, so that the capillary stream is exposed to 

 air on either side. It appears from the researches of M. Bourgery,t that the 

 development of the air-cells continues in the human subject up to the age of 

 thirty, at which time the capacity for respiration is the greatest ; it subsequently 

 decreases, especially in persons who suffer from cough, the violence of which 

 expiratory effort frequently causes rupture of the air-cells, and thus gradually 

 produces that emphysematous state of the lungs which is so common in elderly 

 persons. The power of increasing the volume of air taken in, by a forced 

 inspiration, is much less in the old person than in the child, though the average 

 amount of air inspired may be the same ; hence the young person possesses a 

 greater capacity of respiration, as it were, in reserve ; whilst the old man has 

 little, and is therefore unfit for great exertion. 



526. The lungs are developed, in the first instance, as diverticula from the 



* Philosophical Transactions, 1842. 



j- Archives Generates de Medecine, Mars, 1843. 



