Note 3, p. 188. The other senses, being for, &c.] It may be questioned whether this description is absolutely true, as there are creatures which, although fixed to one habitat, not capable of progression that is, are endowed with all the senses, although it may be in some modified, or less active form than in the higher animals. For "the nervous system has been detected in every division of the animal kingdom, and almost in every class, and it is everywhere connected with sensation and motion."
Note 4, p. 188. Be sensible through a medium, &c.] The medium, that is, made diaphanous and motive by colour or sound, acts, by a succession of undulations, upon the eye or the ear, and finally, through the humours of the former and the air in the latter, upon the sentient part of these organs; so that there is an evident analogy between these undulations and the impulses which maintain locomotion until lost in the state of rest. This succession of impulses may well apply to the changes which, without change of locality, are slowly and silently going on in bodies, and be compared to colouring matter which permeates and gradually combines with each molecule of wax up to saturation; but every substance cannot, of course, be thus affected—a stone, for instance, owing to the condensation of its particles, cannot be made receptive of colour.
Note 5, p. 189. The air is mobile in the highest degree, &c.] The nature and properties of the atmosphere were imperfectly known, as has been said, in the age of Aristotle. It was deemed necessary to sensation that the air should be still (for, when in motion, it was converted, according to opinion, into wind), and as one mass; and as this aggregation of the air could be only over smooth surfaces, the outer coat of the eye (the cornea) seemed, by its smoothness, to favour Aristotle's doctrine, that vision is through a medium, and completed, by refraction, at the bottom of the organ. The medium, set in motion, by colour, was said to give motion, by successive impulses, to the air over the cornea, which communicated the impulse to the organ within; and this superseded the doctrine that vision is produced by rays emanating from the eye. Thus, "the air (that over the cornea), in its turn, sets vision in motion;" but the last clause of the sentence is very obscure, and offers, as some commentators have said, "great difficulties." It may, perchance, be a continuation of the analogy and suggest that, as colouring matter acts, successively, until each particle is saturated, so the impulse is transmitted to the cornea, and finally, from it, to the visual faculty within.
PRELUDE TO CHAPTER XIII.
There were four admitted elements, fire and air, earth and water, from which all things were supposed to be formed; and the object here is to shew that an animal cannot be homogeneous, connot be formed, that is, of only one element. Now, earth was supposed to give solidity, fire to be diffused through all living bodies, and thus there remained only air and water from which to constitute sentient organs; the earth was assigned, however, more particularly to the Touch, but, as Touch is perceptive of other qualities (as hot and cold) besides those of mere Touch, it could not be constituted of that element alone. The organs of relation, sight and hearing, were formed, principally, of water and air, which are the media for the transmission of visual and sonorous impressions; and the smell partook more of fire, as the Touch did of earth, yet not exclusively. The theory is superseded by the increase of our knowledge of external nature, but, in assuming elements, combination and proportion, it seems to typify, as it were, an atomic theory. Hippocrates, also, taught that the human body cannot be exclusively, either of air or fire, water or earth, or any single element; although, he adds, "I do not quarrel with such as think otherwise." Plato, likewise, “derived all things, so to say, from these four elements, in due proportion and relation to one another; so that what fire is to air, that air is to water, and water to earth, and each is, by affinity, united with others, to form whatever is visible and tangible.”
Chapter XIII.
It is manifest that an animal body cannot be simple, cannot, I mean, be only fire or air; for an animal cannot have any other sense without having Touch, and every living body is sensible to Touch, as has been said. But all the elements, except earth, may constitute sentient organs, as these all receive impressions through something foreign to themselves and through media; but the Touch is made sensible by touching bodies, and hence its name. And yet the other sentient organs do perceive by Touch, but then it is through something foreign to themselves; while Touch alone seems to perceive directly, through itself. So that an animal body cannot be constituted of any one of the other elements exclusively, nor can it be formed only of earth; for the Touch is the medium, as it were, for tangible impressions, and the organ is perceptive not only of the distinctions which pertain to earth, but of hot, cold, and all other tangible qualities. Hence it is that we have no feeling in bones, hair, or other analogous parts because they are of earth; and plants for the same reason, being of earth have no feeling. It is impossible then that there should be any other sense without that of Touch; and its organ is neither of earth nor any other element exclusively. Thus, it is manifest that the Touch is the only sense of which animals cannot be deprived without dying; that animals only can possess it; and that it alone of the senses is necessary to animal existence. On which account, other sentient impressions in excess (as those of colour, sound and odour) may injure the organs but do not destroy the animal, excepting it be by chance, as when with sound there is an impulse and a blow, or as when, by visual or odorous impressions, other influences are set in motion, which destroy the animal through the Touch and when savour destroys life it does so by communicating simultaneously a tangible impression. But the excess of tangible impression, whether hot, cold or hard, destroys the animal, because as every impression in excess destroys the sentient organ, so the tangible destroys that of Touch, and it is by the Touch that animal life has been defined; and it has been shewn that an animal cannot possibly exist without the Touch. Thus, the excess of tangible impressions destroys not the organ only but the animal, as that sense alone is necessary to its existence. Animals, in fact, possess, as has been said, the other senses, not merely for existence but, for higher enjoyment: they have sight, in order that, as they live in air and water, in a transparent medium in short, they may see; taste, that by discerning what is grateful or nauseous in food they may have desire for and move to obtain it; hearing, that others may signify something to them, and a tongue that they may signify something to others.
Notes Layout 2 Note 1, p. 192. The Touch is made sensible...and hence its name, &c.] The text refers to etymology to shew, that as, in all times, it had been noticed that the impression upon other senses is different from that upon Touch, it had hence obtained its appellation ἁφή, which, being derived from ἅπτω, (to fasten or bind,) signifies fastening or binding, and so (by touching,) immediate contact; as contact is necessary for the sensation of Touch. This may suffice for the explanation of the term in the original, but it may not, of course, be applicable to its synonym in a modern language, since its origin may be from another idea, and, therefore, a different root.
Note 2, p. 192. And yet the other sentient organs, &c.] It had been proved analogically, that, as bodies in the water are separated by the water, (as was supposed to be proved by their extremities being wet,) so bodies in the air are separated by air, and therefore, that, as no one body is in immediate contact with another body, sensation can be effected only through a medium; and this was supposed to hold good even for the Touch. Thus, the medium, acted upon and acting in its turn, reduces all sensual impressions to the one impression by contact, and this generalisation is supported by some modern writers and regarded as the theory of Sensation. "There may, however, be many other impressions derived from outward bodies, for which the sensitive nerves of the lower animals are adapted, besides those which affect us, and we cannot always be certain of the identity of the feelings communicated to them by organs which appear analogous to our own."
Note 3, p. 193. On which account, other sentient, &c.] This is consonant with the opinion that the Touch is the only sense necessary to animal existence; although the organs of relation are required for the higher forms of being. Thus, impressions in excess upon those organs, whether by colour, sound, or odour, may injure or pervert the senses, but cannot further affect the individual; while tangible impressions, hot, cold, or hard, can together with the sense destroy the animal.
Note 4, p. 193. Animals, in fact, possess...the other senses, &c.] This is referrible, of course, only to the higher orders of animals, as they alone require such organs for the exercise of their faculties, and the enjoyment of their existence. The Tongue is here introduced, whether by inadvertence or in submission to common opinion, as if it were a sense, or the sole organ for speech; and yet, as the chief of the organs for taste and speech, it may be said to constitute one of the distinguishing features of humanity. As no creature, however, is without a tongue, it can scarcely be supposed to be wanting, and yet, as it would not seem to be so essential as some other parts, life might, perhaps, for a time, be maintained without it. But speech is, of course, nowise necessary to life, as the learned commentator observes: "Nam etiam linguæ sermone, si vitam, detractis ornamentis, ad necessitatis angustias redigere velis, vitæ conservatio carere potest."