SECT. IX. PHYSIOLOGY. 85 
contractile power to them; the capacity of the 
auricle is thus suddenly diminished, and _ its 
parietes press with violence on its contents. By 
this contractile effort of the auricle, the blood is 
partly forced back into the venz cave, but by far 
the greater part passes into the right ventricle, 
which is expanded to receive it, and from which 
its return is opposed by the tricuspid valve. The 
blood rushes into the ventricle with great force, 
not simply from the impetus communicated by 
the contraction of the auricle, but also from the 
suction of the expanded ventricle. 
» excevul. In the ventricle, the blood produces the 
same effect as in the auricle, and is propelled 
vehemently into the pulmonary artery. The ex- 
pansion of the right ventricle would suck back 
the blood from the pulmonary artery, did not. the 
semilunar valves, which are bellied out on their 
cardiac aspect, prevent its return. 
excvitt. The pulmonary artery divides into an 
infinity of small branches, which are ramified on 
the air-cells of the lungs, where the venous blood 
is decarbonized and changed to a- bright red 
colour. The blood passes most freely through 
the lungs during inspiration; but as this position 
has been denied, I shall offer some arguments in 
its defence, when I come to respiration. 
cexcrx. The blood, in passing through the 
capillaries of the lungs, parts with a quantity of 
