Popular Lectures on Zoonomia / Or The Laws of Animal Life, in Health and Disease

by Thomas Garnett

With regard to his reasoning faculties, or mind, man is eminently distinguished from other animals. It is by this noble part that he thinks, and is capable of forming just ideas of the different objects that surround him: of comparing them together; of inferring from known principles unknown truths; of passing a solid judgment on the mutual agreement of things, as well as on the relations they bear to him; of deliberating on what is proper or improper to be done; and of determining how to act. The mind recollects what is past, joins it with the present, and extends its views to futurity. It is capable of penetrating into the causes of events, and discovering the connexion that exists between them.

Governed by invariable laws, which connect him with all the beings, whether animate or inanimate, among which he exists, man has certain relations of convenience, and inconvenience, arising from the particular constitution of the surrounding objects, as well as of his own body. These external objects possess qualities which may be useful or prejudicial to him; and his interest requires, that he should be capable of ascertaining and appreciating these properties.

It is by sensation, or feeling, that the knowledge of external objects is obtained. The faculty of feeling, modified in every organ, perceives those qualities for which the peculiar structure of the organ is fitted; and all the various sensations of sound, colour, taste, smell, resistance, and temperature, find appropriate organs by which they are perceived, without mixing with, or confounding each other. External objects, therefore, act upon the parts of the body endowed with feeling, and their action is diversified in such a manner, as to give us a great number of sensations, which appear to have no resemblance to each other, and which make us acquainted with the various properties of surrounding objects.

It would not, however, have been sufficient for man, merely to have possessed this power of perceiving the different properties of the objects which surround him: it was necessary likewise, that he should be possessed of motion, that he might be able to approach or avoid them, to seize or repulse them, as it suited his convenience or advantage. By the extreme mobility of his limbs, he is able to move his body, and transport it from place to place; to bring external objects nearer to him, to remove them to a greater distance, and to place them in such situations and such circumstances, as may enable them to act on each other, and produce the changes which he wishes.

The human body, therefore, may be regarded as a machine composed (besides the moving parts which have formerly been noticed) of divers organs upon which external objects act, and produce those impressions which convince us of their presence, and make us acquainted with their properties. These impressions are transmitted to the sentient principle, or mind; and the faculty we possess of perceiving these impressions has been called by physiologists, sensibility.

Sensation has generally been defined by metaphysicians to be a change in the mind, of which we are conscious, caused by a correspondent change in the state of the body. This definition, however, leaves the matter where they found it, and throws no light whatever on the nature of sensation; nor can we say any thing more concerning it, than that, when the organs are in a sound state, certain sensations are perceived, which force us to believe in the existence of external objects, though there is no similarity whatever, nor any necessary connexion, that we can perceive, between the sensation and the object which caused it.

All the different degrees of sensation may be reduced to two kinds: pleasant and painful. The nature of these two primitive modes of sensation, is as little known to us as their different species: all that can be said, is, that the general laws by which the body is governed, are such, that pleasure is generally connected with those impressions which tend to its preservation, and pain with those which cause its destruction.

In a general point of view, sensibility may be regarded as an essential property of every part of the living body, disposing each part to perform those functions, the object of which is to preserve the life of the animal. Sensibility presides over the most necessary functions, and watches carefully over the health of the body: she directs the choice of the air proper for respiration, and also of alimentary substances; the mechanism of the secretions is likewise placed under her power; and in the same way that the eye perceives colours, and the ear sounds, so every animated and living part is fitted to receive impressions from the objects appropriated to it.

That every part of the animal is endowed with sensibility, is evident from a variety of facts, particularly from the action which follows when a muscle taken out of the animal body is irritated by any stimulus: this is evident, by a variety of facts mentioned by Whytt, Boerhaave, and others, which show, that parts recently taken from the animal body retain a portion of sensibility, which continues to animate them, and render them capable of action for a considerable time.

The primary organ of sensation appears to be the brain, its continuation in the form of medulla oblongata and spinal marrow, and the various nerves proceeding from these; and it seems now generally agreed, that unless there be a free communication of nerves between the part where the impression is made, and the brain, no sensation will take place; for instance, if the nerves be cut or compressed.