THE PHYSIOLOGY OF THE BLOOD. 
515 
monary artery have two terminations, one on the surface of the 
air-cells into the minute origins of the pulmonary veins, and 
the other by exhalent orifices on the inner surface of the air-cells ; 
while, on the other hand, the pulmonary veins not only arise from 
the extreme branches of the pulmonary arteries on the surface 
of the cells, but by extreme radicals from their inner surface.” 
l do not agree with this opinion ; for were this the case, I think 
we should have more frequent hemorrhage from violent exertion of 
the lungs ; especially as these vessels contain coloured blood, 
and consequently the orifices must be comparatively large to 
admit the globules ; besides, there appears no necessity for it, 
as the membrane lining the air-cells, like other membranes, will 
transmit gases freely. 
The atmosphere, in its passage through the lungs, is robbed 
of part of its oxygen, and receives an increased quantity of 
carbonic acid gas and watery vapour ; it also undergoes other 
changes dependent on circumstances. Whence comes the 
carbonic acid gas? On this point there are different opinions. 
Dr. Prout says, “ we are aware no gelatine exists in the blood, 
but a great deal of albumen, which contains three or four atoms 
per cent, more carbon than gelatine, therefore, as gelatine enters 
very largely into the composition of animal substance, more 
especially into skin and bone,” he gives this as one source of 
carbon, which, uniting with the oxygen in the blood, forms the 
carbonic acid that is proved to exist in venous blood. One great 
objection to this is — in the removal of parts, whence is the carbon 
that forms the gelatine changed into albumen, as no gelatine 
exists in the lymphatic absorbents ? 
I am inclined to believe the excretion of carbonic acid gas 
is a vital property of the lungs, effected as all other excretions, 
and because it is prejudicial to animal life. The researches of 
Majendie and others proved the existence of carbonic acid gas in 
almost the whole extent of the alimentary canal, formed in the 
process of digestion. Dr. Edwards justly observes, “ its being in 
contact with the whole mucous surface of this canal, a part must 
be absorbed.” As a proof of its capability of absorption, the 
fact of water impregnated with carbonic acid, and drunk in 
sufficient quantity, producing symptoms of asphyxia, is import- 
ant. The carbonic acid excreted by the skin appears to be 
immediately formed there ; the skin being principally composed 
of gelatine, and the minute capillary tubes of the arteries termi- 
nating on its surface by exhalent orifices, the carbon given off in 
the deposition of gelantine uniting with the oxygen of the 
atmosphere, forms the carbonic acid which is evolved. 
The atmosphere is composed of oxygen -i, nitrogen with a 
