. VI. MECHANISM OF RESPIRATION. 1 19 



and, lastly, the physical properties of the air itself, are the 

 causes of the respiratory movements. 



The whole thoracic cavity dilates during inspiration, and 

 the air rushes into the bronchia ; whereas during expiration, 

 this cavity contracts, and the cells of the lungs, being 

 elastic, resume their primitive volume, whereby the air, be- 

 ing thus compressed, and possessing an elasticity more or 

 less great, in proportion to the degree of heat communicated 

 to it by the lungs, is expelled. The simple action of a pair 

 of bellows shows you the whole mechanism of the respira- 

 tory movements. 



In fishes, this motion takes place without the co-opera- 

 tion of the ribs. The branchial arches open, the lamellae 

 separate, and the contact between them and the water thereby 

 takes place. They then close, and the water escapes by 

 the branchial fissure, which remains open until the opercu- 

 lum falls. 



In the lower animals respiration is less energetic, and the 

 motions of inspiration are almost involuntary. In the an- 

 nelides and mollusca, the current of water, in which the air 

 is dissolved, seems aided by the vibratile movements of 

 the cilia placed on the branchiae of these animals. 



Jlir respired. A man, at one respiration, introduces into 

 his lungs about 20 cubic inches, or rather more than 

 half an imperial pint, of atmospheric air ; the air expired 

 contains usually from 3 to 5 per cent, of carbonic acid ; but, 

 after a very deep expiration, as much as 6 or 8 per cent. 

 At the same time the inspired air has lost from 4 to 6 per 

 cent, of its oxygen. 



The numbers I have quoted have been selected from 

 many others as being those which appear most worthy of 

 confidence. From them it is easy to calculate the quantity 

 of oxygen which a man absorbs by respiration in a day ; 

 assuming that he makes from 15 to 20 inspirations in a mi- 



