18 PRACTICAL ORNITHOLOGY. 



tissue, which permeates tlie body of all birds, although more 

 developed, and somewhat differently arranged in certain fami- 

 lies. The air admitted by inspiration passes into the lungs, 

 where it produces the usual effect upon the blood there, enters 

 the cells communicating with them, and makes its way nearly 

 to all parts of the body. Although there are of course a regular 

 influx of air into the lungs, and an efflux from them, and the 

 oxygenation of the blood there is manifested by its florid tint ; 

 yet it does not appear that a similar alternate motion takes place 

 in the vast series of cells communicating with the lungs, and 

 it is probable that in passing through the latter the air loses the 

 greater part of its alterative power, for the minute vessels ramify- 

 ing upon the cells distributed through the body do not assume a 

 brighter hue than those remote from them. One obvious effect 

 however is that the air thus introduced into the cells tends to 

 render the body specifically lighter, and this effect is promoted 

 by the expansion which it undergoes from the heat of the animal. 

 The most obvious change produced upon the blood by re- 

 spiration is its assumption of a bright red colour, in place of 

 the purplish-red which it presented before entering the lungs. 

 Chemical investigation has further disclosed to us that it im- 

 bibes oxygen, and gives out carbonic acid gas, together with 

 water in the state of vapour. Atmospheric air is composed 

 of about twenty parts of oxygen gas, seventy-nine of nitro- 

 gen gas, one of carbonic acid gas, and a trace of hydrogen. 

 Arterial blood has been found to contain more oxygen than 

 venous blood, while the latter contains a larger proportion of 

 carbon, and the proportions of nitrogen and hydrogen are 

 nearly the same in both. Now, as during respiration oxygen 

 is extracted from the air, and carbon from the blood, and 

 since carbonic acid consists of oxygen and carbon, the por- 

 tion of carbonic acid exhaled from the lungs is the result 

 of the combination of the oxygen which disappears during re- 

 spiration, with the carbon contained in the venous blood, and 

 which has accumulated in it in consequence of the abstraction 

 of its oxygen, hydrogen, and nitrogen, by the processes of secre- 

 tion and nutrition. But, letting alone the theories that have 

 been proposed on this subject, it is enough for our present pur- 



