But there being always some of the more tenuous parts of the air left, so that all of it is not exploded, to that which there remains the more ponderous external air with equal violence is forced; and this he compares to cupping-glasses.

All spontaneous breathings are formed by the contracting of the smaller pores of the lungs, and to the closing of the pipe in the neck; for these are at our command.
Herophilus attributes a moving faculty to the nerves, arteries, and muscles, but thinks that the lungs are affected only with a natural desire of enlarging and contracting themselves.

Farther, there is the first operation of the lungs by attraction of the outward air, which is drawn in because of the abundance of the external air.

Next to this, there is a second natural appetite of the lungs; the breast, pouring in upon itself the breath, and being filled, is no longer able to make an attraction, and throws the superfluity of it upon the lungs, whereby it is then sent forth in expiration; the parts of the body mutually concurring to this function by the alternate participation of fulness and emptiness.

So that to lungs pertain four motions--first, when the lungs receive the outward air; secondly, when the outward air thus entertained is transmitted to the breast; thirdly, when the lungs again receive that air which they imparted to the breast; fourthly, when this air then received from the breast is thrown outwards.