_Of the External Senses._
Feeling is the universal sensation of the nerves, on which all other
senses depend, as the subtile spring of life itself. What it is, and
whence it proceeds, has already in the foregoing been explained; namely,
a sensation of the nerves, by which we perceive an idea of _hard_ or
_soft_, _wet_ or _dry_, _hot_ or _cold_, or in general terms, _pleasure_
or _pain_. The peculiar seat thereof seems to be placed at the ends of
our fingers; whether this is the absolute seat ordained by nature, or
rendered so by custom, is not my business here to enquire; perhaps, both
conjoin to make it there most perfect, as best suited for that purpose.
_Taste_ is the next sensation of the nerves, regarding an immediate
approach and action of substances themselves, in order to distinguish
betwixt the nature of foods, by which we are enabled to form an idea of
_salt_, _sweet_, _sour_, _bitter_, _sharp_, or _mild_; which according
to their mixture and proportion, render _taste_ agreeable or
disagreeable. Its seat is principally in the tongue; but that it extends
to the very stomach itself, is evident, from the immediate consent of
parts; namely, that what is disagreeable to the palate, will ever be
nauseous to the stomach: Of which I shall say more hereafter.
_Smelling_ is that sensation given to the animal creation, as a friendly
messenger of taste, in order to enable us the better to chuse our food.
It is actuated by means of the _olfactory nerves_ spread in the
_pituitary membrane_ of the nose, and has an immediate communication
with the brain; which nervous plexus is actuated by odorous particles,
exhaling from substances, by which the olfactory nerves are stimulated
agreeably, or disagreeably, as the first intelligence to the palate, or
to the whole consent of the nervous system.
_Hearing_ is that sensation of the nerves which is occasioned by the
vibration of the air, and forms the sound, which enters the cavity of
the ear, and tremulates and puts in action, the organs of hearing. The
ear is perhaps as curious a piece of mechanism to our conception, as any
part whatever we shall meet with in anatomy.
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.
Now the act of vision is performed thus: The convex transparent part of
the _cornea_ faces at once every object which comes within the direct
lines thereof, and this is termed a _full view_.[8] This view enters
through the aqueous humour, where the whole is contracted according to
its convex and concave form, so as to make it fit to enter the
_pupilla_, which is a muscular continuation of the _sclerotis_, and
called the _uvea_; and from thence the whole view passes through the
crystaline lens, by which the view is again magnified, and passed
through the vitrous, and the whole depicted on the _retina_, in the same
concave curve, and at equal angles in which it was first received, and
apparently the same magnitude; thence representing the objects in their
natural perfection, figure, colour, magnitude, and motion.
Footnote 8:
It is the general received opinion, that objects are reversed in the
representation on the _retina_ of the eye. This is however a mistake;
I hope on another subject to prove this in a full and satisfactory
manner.
When, however, an object is singled out of the whole view for particular
attention, the _uvea_ then contracts or dilates, which, together with
the muscular action of the whole ball, proportions all the humours
suitable to the magnitude and distance of the object under examination,
and the quantity of light; and to make the eye more capable, the lids
are frequently moving, in order to moisten and varnish, as it were, with
a liquid pressed from the innermost gland, called _canthus major_, to
keep the surface smooth, and assist its transparency. If this intense
looking is continued beyond the strength of this noble organ, it relaxes
from its proper destined expansion, and the objects are rendered
obscure.
Darkness is ease and rest to the eye, because it is then out of action;
whilst light always keeps it in exercise; And in diseases of the eyes,
light should be kept away as much as possible.
Here then I shall beg leave to conclude this short description of the
wonderful structure of our frame; which when we duly consider, will
never fail of filling our breasts with a sense of admiration; and induce
us to lift up our souls with reverence and gratitude, to that
incomprehensible GREAT CAUSE, by whose wisdom we are formed; and by
whose goodness we exist.
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