Experience has sufficiently informed us that the organs of sense must be in a healthy state, in order to the due conveyance of perception. When the function of any organ is altogether defective, as when a person is born blind, he is cut off from all perception of light and of visible objects. If by nature deaf, from the intonation of sounds; and many unhappy instances of such connate defects abound among our species. In one particular subject, both these defects existed from birth; so that the sum of his intelligence was conveyed by the touch, smell, and taste, or in other words, his mind was exclusively composed of the perceptions he derived from these senses. This case will be more particularly noticed in a subsequent chapter. The alterations which take place in the state of our perceptions from a morbid cause, are generally known. Thus a person labouring under a catarrh, will be unable to detect the odours which certain substances communicate in a healthy condition of his olfactory organ. In fever excited by a disordered stomach, the taste will become vitiated, and the partial obstruction of the ear by accumulated wax, will impress him with the bubbling of a pot, the singing of birds, or the ringing of bells.
The same law that produces fatigue in a muscle from exertion, appears to obtain in the organs of sense. If they be excited by their appropriate stimuli too violently, or for a too long continuance, fatigue or languor is produced, their percipience is diminished, or confusedly conveyed; and they require a period of rest for their refreshment.
As we advance in our enquiries into the nature of perception, it will be evident that we cannot long continue to treat of it as a simple act, or as a distinct faculty. The organs by which we obtain our different perceptions are not insulated parts, but communicate with a substance, termed the brain, and which is continued through the vertebral column. The ultimate expansion of a nerve of sense, has been termed its sentient or percipient extremity; and where it is united to the brain, its sensorial insertion. If we were to divide the optic nerve where it passes into the foramen, taking care to leave the apparatus of the eye uninjured, the visual organ would be deprived of its function, and the person or animal would be completely blind of that eye; so that a communication with the brain is necessary for the purpose or act of perception. As therefore the union of the nerve with the brain is indispensably necessary for the purpose or act of perception, we are naturally led to inquire into the properties of this substance, termed the brain. Before we proceed to this part of the subject, it will be proper to notice a fact which is of frequent occurrence. In amputations of the thigh, at the moment the femoral nerve is divided, it often occurs that a pain is distinctly felt in the toes; and after the limb has been removed, even for many months, the same painful feeling of these lost extremities is occasionally experienced. This circumstance would render it probable that the larger branch of the nerve becomes itself impregnated with the sensation it transmits: indeed it is a continuation of the same substance, from its sentient extremity to its sensorial insertion. This intimate union of nerve and brain may be further illustrated: it has been already noticed, that a morbid state of the organs of sense will convey inaccurate perceptions; and it is equally certain, that disease of the brain, will excite phantasms, which appear as realities to the sensitive organs.