in beasts, is the same with the secundines, or after-birth in women.

HEARING the sense whereby we perceive sounds.

The organ of hearing is the ear, and particularly the auditory nerve and membrane. See Anatomy, p. 295.

This membrane, in the various degrees of tension and relaxation, adapts itself to the several natures and states of sonorous bodies; becoming tense for the reception of acute sounds, and relaxed for the admission of grave sounds. In short, it is rendered tense and relaxed in a thousand different degrees, according to the various degrees of acuteness or gravity in sounds.

Sound, then, is in effect nothing but a certain modulation of the air, which being collected by the external ear, passes through the meatus auditorius, and beats upon the membrane of the tympanum, which moves the bones in the tympanum; these move the internal air, which finally communicates the motion to the auditory nerve, in the labyrinth and cochlea; and according as the vibrations are quick or slow, the sound is either acute or grave.

It deserves observation, that though the air be the usual matter of sounds; so that if a bell be hung in vacuo, it will not be heard at all; yet most other bodies, properly disposed, will do its office, only some more faintly than others. Thus a sound may be heard through water, or even through earth, of which there are various instances.

As the sight is assisted by spectacles, or other glasses; so the hearing is enlivened and rendered quick, by means of acoustic instruments; which are of various figures, but for the most part bear some resemblance to a trumpet, diverging and growing wider towards the external mouth.