To the organs of hearing we must reckon the external and internal _auditory_, divided by a thin membrane, which is a thin spreading of nerves, called the _drum_; within this, or in the internal cavity, we observe, 1st. Four little bones called, from their forms, the hammer, _malleus_; the anvil, _ancus_; the stirrup, _stapes_; and roundling, _orbicularis_. 2dly. Their peculiar muscles. And, 3dly. Two considerable cavities, the _labyrinth_ and _aqueduct_. How hearing is actually brought about, Anatomists have not as yet agreed; and which would be too tedious here to enquire into. That the little hammer is perpetually moving on the _tympanum_, according to the different sounds, may be one part; which, however, is greatly assisted by the other organs.
The _labyrinth_, with many turnings and windings, penetrates through the skull and forms part of the aqueduct, by which the sound is modulated and magnified, so as to cause the distinction between them, and convey the idea of their sonorousness to the common receptacle of sensation.
_Seeing_ is that great faculty by which the form, colour, and motion of objects is represented to our ideas. The principal conveyer of this wonderful sensation, is the _optic nerve_; the eye is a telescope finished to the greatest perfection; or like a _camera obscura_, where objects are pictured to the utmost nicety, and a due proportion observed in their nature. It is a globe enrolled with a strong tendinous coat, and filled with three kinds of humours perfectly clear and transparent, which differ in nothing but their various degrees of liquidity and form. Each of these humours is enclosed in a delicate thin transparent membrane. The external coat of the eye is called the _cornea_, from its resemblance to horn, being a continuation of the _dura mater_ and _pia mater_; and is every where opaque, except at the entrance of the light, which is called _pupilla_, and projects somewhat beyond the spherical form of the eye, similar to the crystal of a watch. Inside the _cornea_ it is lined with a spreading of the optic nerves, called the _retina_, where objects are reflected; by which the ideas are immediately conveyed to the brain.
The humours are the following: The _aqueous humour_, the most external, which is designed for collecting the rays of light; the second is the _crystaline humour_, which is in the form of a magnifying lens, and invested with a subtile transparent tegument, and so curiously fixed, as to be contracted and dilated, according as the objects are near, or removed from the eye; and accordingly the crystaline humour requires to be flattened or thickened; the last is the _vitrous humour_, which fills the remaining cavity of the eye; this humour serves to spread the objects again on the _retina_, and arrange them in their natural order, so that the picture of the objects viewed, may be preserved in its due proportion and regularity.
