1723. They constitute the interior membranes of man, which are spacious, and on this account there are such vast numbers or troops of them; for the membranes are not otherwise, or do not perform any other use, than to act as passive forces, and to do whatever the active forces impress upon them. Concerning these spirits, it is believed that they constitute the pleura which surrounds the chamber of the thorax, and insinuates itself through the pericardium into the regions of the lungs, and that they thus pass into the pharynx and larynx, which is an organ of speech, as are also the lungs that are encompassed by the pleura, and to which it constitutes the covering.