Sir Everard Home’s examination into the structure , &c. 59 
oxygen and nitrogen, mixed together, is received into the 
lungs, and in every expiration, the same volume is returned, 
measure for measure exactly, with this only difference, that 
what entered as oxygen is returned in the form of carbonic 
acid gas, which, according to their theory, proves that no part 
of the inspired atmospheric air has been retained in the lungs, 
but a quantity of carbon, equal to that of the oxygen inspired, 
has been extracted from the blood by the oxygen, making it 
become carbonic acid gas. 
Nothing could be more ingenious than this theory, were it 
supported by the structure of the lungs themselves, and it 
could be proved that the blood requires no other changes for 
its purification ; since all the leading facts on which it is 
founded, are completely established upon the firm basis of 
experiment. 
When this theory was formed, the structure of the air 
cells of the lungs had never been examined, the more minute 
structures in animal bodies being at that time considered 
beyond the reach of examination ; and it is the object of the 
present communication, to bring forward an explanation of 
the mechanism of the cells of the lungs, as well as of the 
different distributions of the vessels that ramify through those 
organs, acquired from Mr. Bauer’s microscopical observa- 
tions, and to see how far they are fitted for the process, which 
by this theory is allotted to them. 
In this investigation, I began by an enquiry into the circula- 
tion of the blood through the lungs, in the labour of which 
I have been very ably assisted by Mr. Russell, a very intel- 
ligent student of St. George’s Hospital, at present filling the 
highly respectable and important office of senior House 
