86 ANALYTIC SECT. IX, 
carbon, some water, and a very little hydrogen; 
and is thus changed from venous to arterial blood ; 
the oxygenation which it was supposed to 
undergo, by coming in contact with the air, is a 
very pretty theory, but is unhappily not true. 
The four pulmonary veins which convey the blood 
back to the heart, are formed by the junction of 
numerous small branches, which must. in some 
degree be influenced by the motion of the lungs. 
cc. The left auricle and ventricle are sepa- 
rated by the mitral valve, and follow the same 
order of expansion and contraction which has been 
described. The systole and diastole of the two 
auricles are synchronous, and the two ventricles 
expand and contract at the same moment; but 
the auricles are expanded when the ventricles are 
in a state of contraction, and vice versd. 
ccr. It is evident that the expansion of the 
ventricles does not operate solely on the blood con- 
tained in the auricles; and were it not for the 
obstruction of the semilunar valves at the orifices 
of the aorta and pulmonary artery, the suction of 
the ventricles would draw the blood from them 
as readily as from the auricles; the diastole of the 
ventricles must therefore subject the semilunar 
valves to considerable pressure from the reflux of 
blood at the commencement of the aorta and 
pulmonary artery. The eddy thus produced in 
the blood, at the cardiac extremities of these great 

