Amongst many tribes of animated beings, sensation is limited to the purposes of locomotion and nutrition. With them this material property is not improvable, but continues stationary during their existence. With other animals, sensasation is capable of a more exalted manifestation: in its subserviency to instinct, it is perhaps most perfect amongst insects. In them, however, it displays no evidence of being tributary to reasoning. In the higher vertebrata, it serves both faculties, apparently in the inverse ratio of each other.* In the human subject, its education is the unconscious labour of infancy :—it then answers an instinctive purpose. Its subsequent services are chiefly intellectual. It will be admitted that, amongst the lower tribes of animals, whose organism is incomparably inferior to our own, sensation is a function or property of matter—a consequence of organization. The instinctive actions of childhood, and of reasonless humanity, are to be explained upon this principle. And even when in the rational man, sensation performs its highest office as the servant of intelligence, it must be conceded that, whatever is animalt in the action is the offspring of materiality. The action may be reasoned upon with whatever acuteness the mind is capable, but the original suggestion is neither more nor less than the function of the matter transmitting it. It is invariably connected with, and dependent upon, a change of state in some part of the nervous system. In what it essentially consists, and where it resides, are ultimate facts in physiology with which we shall perhaps never be acquainted. The former question was referred by Newton and Hartley to vibrations; by Darwin to contrac * Thus birds, and most of the mammalia, have senses more acute than our own. also applies to man in his savage, compared to his civilized, state. + The term animal is here used in contradistinction to mental or spiritual. This tions and elongations; and by later writers, successively, to electricity, galvanism, and magnetism. All we can say is, that, one theory is as fertile in its conjectures, and as faulty in its conclusions, as another. The seat of sensation has been variously referred to the pineal gland; to the pia mater; to the fluid in the ventricles of the brain; and to the medulla oblongata. Surgical pathology sometimes tells us that the assistance of the brain is necessary to the ultimate action upon which sensation depends; but it does not prove that this act occurs in the brain. The observations of Gall and Spurzheim deny to this organ any share in the function of sensation. The most probable opinion is, that, "the process from which sensation results is actually performed within the particular nervous structure connected with each organ of sense."*+ * Pritchard on a Vital Principle, p. 166. Thus most of, perhaps all, the sensations, are suggestive of one concomitant idea -locality. We do not confound cold feet with cold hands; we know that the sense of hunger is not in the nose; and that the function of hearing is not in the eyes, nor that of seeing in the ears. The magnetisers indeed tell us of people reading with the backs of their heads, but these are wonders, not of physiology, but of the age we live in. Let us ask Lucretius for a lesson, and then raise a blush at our own credulity, and a sigh for our civilized impostors. An poterunt oculos aures reprehendere? an aureis Can sight correct the ears? can ears the touch? IV. WE learn from the preceding observations that, sensation is essentially a material function, excited by the influence of external agents upon the sensual organs. The action of the organs of sense leads to SENSATION and suggested IDEASthe mode is called PERCEPTION, which may be denominated the CONSCIOUSNESS of sensation. Through our senses also we are led to CONCEPTIONS, it may be of a lofty order, developed through abstract REFLECTION. Ideas are the offspring of perception; they are permanent, and though not always present to the mind, are capable of being recalled by ASSOCIATION, and thus is constituted the faculty of MEMORY. When not attended by a conception of the original impress and influence upon our senses, these associations are denominated PHANTASIES. The association of ideas, though a mental phenomenon, is absolutely dependent upon that action of the nervous system which is the consequence of organization: the primitive influence is through the senses; it involves also an organic action of the brain; and it is not improbable that the recalling of every idea requires for its precursor or concomitant, a change in the material state of this organ. "The organic action of the brain seems to be as requisite to the recalling of an idea into the mind, as the movement of the string of a harp is to the generation of a corresponding musical tone."* (Pritchard.) This, indeed, is the basis of the elegant theory * The sense of this passage is very similar to the theory of Harmony of Aristoxenus, as thus explained by Lactantius: sicut in fidibus, ex intentione nervorum efficitur concors sonus atque cantus, quem musici harmoniam vocant; ita in corporibus, ex compage viscerum et vigore membrorum vis sentiendi existit. "As, in musical instruments, an accord and consent of sounds, which musicians term Harmony, is produced by the due tone of the strings; so in bodies, the faculty of perception proceeds from the due connexion and vigour of the members and organs of the body." of Hartley, which, despite its verbal extravagance, will be found to be correct if we regard the excess of its materiality in the light of metaphor. According to this doctrine, on the impression of external objects upon the minute nervous fibrilla of the organs of sense, the whole length of the affected nerve becomes agitated with a vibratory motion, which is hereby propagated immediately to the white medullary substance of the brain, constituting the direct instrument by which the existence of objects is presented to the mind. Every vibration upon the brain leaves behind it a certain mark or vestige of itself, which may figuratively be regarded as a type or image, and, in this sense, be denominated a simple idea of sensation; and the more frequently these vibrations are renewed, or the more vigorously they are impressed, the stronger will be their vestiges or sensible ideas. By frequent repetition, moreover, these sensory vibrations produce in the medulla of the brain a disposition to vibratiuncles, or miniature vibrations corresponding to the parent impression, and on the existence or excitation of these depend both the imagination and memory. Any orders of sensation, by being associated together a certain number of times, obtain such an influence over each other, that either of them, when impressed alone, possesses a power of exciting the vibratiuncles or ideas of all the rest. These associations are divided into synchronous and successive; and all our simple and complex ideas are ascribed to the influence of this principle, or habit. The sight of one part of a large building suggests instantaneously the idea of the remaining parts by a synchronous association of the parts; and the sound of the words, which begin a familiar sentence, brings to remembrance the remaining words in their order of successive association. The foundation of the Hartleian theory is strictly philosophical, though it has a few errors of anatomy which have been fully exposed by the assistance of the microscope. But in the extended sense in which it was applied by its accomplished author, we must admit that it represents man as little better than an automaton, a passive subject of external impulse. Had it not been employed in the explanation of the higher operations of mind, as volition, judgment, and imagination, which it never can by possibility reach; but confined to the elucidation of sensory, instinctive, and other actions springing directly from material organization, it would have been, and for ever, the basis of all rational metaphysics. Pathology daily confirms its exposition of the phenomena of memory. How numerous are the proofs that this function is dependent upon the organic structure of the brain! Not otherwise, indeed, can we account for the fact that, things are never lost from the mind, having once impressed it. The characters are indelible, though the faculty of association may not be able to render them apparent. Thus it is that concussion or inflammation of the brain has often left its subject rational, but seemingly robbed of all his acquirements, until, after the lapse of weeks, months, or years, the tide of previous knowledge, has again, mysteriously and in a moment, flowed upon its conscious possessor, not a wavelet wanting. So in the delirium of fever, and in the visions of death, things forgotten since childhood, crowd upon the busy recollection. Languages that have not been articulated for years, are often fluently spoken in the hour of dissolution. I was once permitted the melancholy privilege of attending the death-bed of a dear friend. He was a good man, and a gifted scholar. Though he had little studied ancient literature for years, and never referred to it except when invited, yet, in his dying moments, the only language he spoke was Latin. |