SECT. IX.
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.
[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.