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two auricles, where, by its connexion, it furrounds 
the two venas cavae: hence, pafling along the arch 
formed by the aorta, it defcends to the pulmonary 
artery, and continues round the orifices of the pul- 
monary veins, firmly attached to thefe feveral parts - 
in its paflage. By thefe connexions, thefe parts are 
all fixed in their feveral fbations, incapable of fepa- 
rating from each other, or fhifting their fituations, 
however they may happen to be compreffed. The 
pericardium is generally laid to ferve as a defence to 
the heart ; but that defence feems to confift chiefly, 
in preventing the right auricle from being ftretched 
by the depreflions (or complanations) of the dia- 
phragm, in hunger and infpiration, and, by its bear- 
ing firmly againfl: the fides of the auricles, to fupport 
and flrengthen them againfl too great diftenfions : 
for the cavity of the pericardium feems to be but little 
more, than commenfurate to the bulk of its contents, 
when one half of them are filled, and the other half 
empty. This will appear, upon endeavouring to fill 
the heart, with its auricles, and its two great arteries, 
with wax, at the fame time, while it is inclofed in 
the pericardium j in which experiment, one or other 
of thefe cavities will be found to have been lo com- 
prefled by the pericardium, as to have refufed a free 
admittance to the wax, and will, therefore, be found 
proportionally empty. 
The inelaftic texture, connexions, and capacity, of 
the pericardium, being thus Hated, let us now con- 
fider the order, in which the feveral diftenfions mud 
have arifen, in the two great arteries and cavities of 
the heart, with the neceffary effedts of thole diften- 
fions 
