The Mosaic History of the Creation of the World / Illustrated by Discoveries and Experiments Derived from the Present Enlightened State of Science; With Reflections, Intended to Promote Vital and Practical Religion

by Thomas Wood

Thus far the absorbent vessels have been described, as employed in taking up fluids only. The action of the absorbent system, however, is not considered as confined to the fluid parts of the body; there are a variety of instances, in which the most solid parts appear to be removed by the absorbents. Thus when a tooth is extracted, or drops out in old age, its bony socket is removed by the action of the absorbents. The pressure of a pulsating tumor, called aneurism, against the ribs, or thigh bone, has produced their removal in the same way. These are considered as instances of solid matter being removed by the absorbent vessels, from internal parts of the body, without any external opening. It is, however, a matter of doubt, which we cannot at present discuss, whether a bone is broken down by the absorbents themselves, so as to be removed in small particles; or whether, as is more probable, its presence or irritation (as an extraneous body) produces the secretion of a fluid, similar in its properties to the gastric juice, by which it is first reduced into minute particles, or entirely dissolved, so as thus to enter the absorbent vessels.

Another important part of the office of these vessels, is to model the shape of the body, and to concur with the action of the blood-vessels in regulating its growth. For the human body does not, like a marble statue, constantly contain the same identical particles in its composition. As the stream of a river is formed of a constant succession of aqueous particles, sometimes increasing, sometimes diminishing its natural bulk; so the human body is constantly undergoing an imperceptible change of parts. The absorbents, by their action, remove exhausted particles, whilst the arteries form from the blood an adequate supply of new parts. When these two powers are equal, the body continues of the same bulk; when from disease or contingent circumstances, the one or the other predominates, the body increases in growth, becomes corpulent, or emaciated.

Thus we have seen a variety of organs necessary to carry on the functions of perfect animals: these, however, are inert, and incapable of motion in themselves. Hence a nervous system becomes requisite, which may excite and influence the whole. We find in man, and quadrupeds generally, the nervous system placed principally in the brain and spinal marrow; from these sources, the nerves are distributed like white cords, and pass in various proportions to the different parts of the body, conveying the excitements of the brain.

One of the most important excitements conveyed from the brain, through the medium of the nerves, is volition; by this means the muscles become obedient to the will, and perform the voluntary actions of animals. If, for instance, I wish to take up a pen, I exert my volition towards the action, and the consequence of this is, that the muscles employed in the action, are stimulated to contract, from a peculiar excitement being conveyed to them from the brain, through the medium of the nerves. We are totally ignorant, however, of the state of the brain, whilst giving out the excitement, or the change which takes place in the nerves whilst conveying it. We know, however, that the brain may be rendered incapable of giving rise to the excitement, and it may be arrested in its progress down the nerves by artificial means. If a ligature be applied upon a nerve by tying a piece of thread round it, the nerve is rendered incapable of transmitting the excitement, so as to produce motion in muscles. The same state is frequently produced in the brain and nerves, by the disease called palsy, or by fractures of the skull. There are also various excitements passing from the brain to the vital organs of the body, whose actions are not regulated by the will, and are therefore called involuntary, or automatic actions, as circulation, parturition, &c. Thus if a person have ever so strong a desire, he cannot make his heart beat more frequently; nor can he prevent it from beating more frequently, if any one should put him in bodily fear; although the heart is formed of muscular flesh, similar to the muscles, which he can command in his arm. The reason of this is, that the nerves of the heart cannot convey the influence of volition; for the wisest reasons the heart acts without it.

It is also necessary for various influences to be communicated from external objects to the brain, to keep up a correspondence between animals, and the material world around them, and to communicate those impressions from which the brain is afterwards to carry on its functions. As the parts formed for this purpose differ from ordinary parts of the body, in having a larger share of nervous influence given to them, they have been called the organs of sense, which in an anatomical point of view, may be said to be five in number, the eye, the ear, the tongue, the nose, and the skin.

In the eye, we discover a most accurate optical instrument, adapted to converge the rays of light at its posterior part. It is composed of a spherical box, containing transparent media of different densities, by which the rays of light are conveyed to a point, so as to impress a minute image of the visible appearance of external objects upon the retina or expansion of the optic nerve, by which the impression is conveyed to the brain, so as to bring us acquainted with external objects.

The ear is formed to receive impressions from bodies in a state of vibration, which are conveyed to the brain by an apparatus composed of various substances, and eminently calculated to transmit the slightest tremors. The vibrations of the air, for instance, first strike the drum of the ear; are thence communicated to a delicate chain composed of four minute bones. By these the vibration is increased, and transmitted to a fluid, contained in several small winding canals, in which the delicate filaments of the nerves of hearing are arranged, so as to transmit the impressions they receive from the surrounding fluids, and produce in the brain the perception of sound; these two senses, by the infinitely varied modification of their impressions, convey a prodigious supply of materials for the action of the mind.

The organ of touch is next in point of importance; it has its seat in the extremities of the nerves distributed over the skin, and is the only sense which belongs to every class of animals. This organ gives rise to sensations, which have no natural alliance with each other. By this sense we compare different degrees of temperature with each other; from this we derive our idea of distance between bodies; of their tangible figure, of their roughness, smoothness, hardness, and other qualities, from the relative position with respect to ourselves, or the degree or kind of resistance they offer. And, when man has been deprived of his communication with many external objects, by the loss of vision, we find the organ of touch gradually encroaching upon the function of the eye, and from attention to its finer impressions, becoming, through the education of necessity, a much more extensive source of information. As an instance of this, I may adduce Mr. Gough, who can accurately distinguish the color and character of flowers, by the nice sense of touch possessed by the tip of the tongue.

The other senses may be said to be of less importance. The nose affords a passage for the air to the lungs, and is impressed by the odorous particles of bodies diffused through it, and, whilst it thus occasionally administers to our gratification, it gives us notice of the presence of those aeriform fluids which are noxious to respiration. Like the organ of taste, which is impressed by sapid bodies, it has a peculiar sympathy with the stomach; thus the taste, or smell, of any disagreeable substance, very commonly excites sickness and vomiting.

Thus each of the organs of sense are formed in a peculiar manner, and are supplied with nerves of a peculiar structure, which are capable of being excited by certain impressions only, so as to give rise to sensation. The odorous particles of bodies, for instance, if applied to the nerves of the nose, excite an impression, which, when conveyed to the brain, gives rise to the perception of smell; but, every one knows that they produce no such effect when applied to the nerves of the skin. In the same way, the rays of light applied to the nerves of the eye produce vision; but, no such effect takes place when they impinge upon the tongue.—Each of the organs of sense then possess a peculiar modification of nerves, which are excited by appropriate impressions.