SENSATION. — (Fr. Sensation ; Germ. Empfindung.)— The improved state of our knowledge of the physiology of the ner vous system makes it imperative that phy siologists should adopt and adhere to a pre cise definition of the term which forms the heading of this article.
Perhaps the simplest definition of sensation which:can be given is the following ; namely the perception by the mind of a change wrought in the body. According to this definition, then, sensation involves, first, a bodily change, from some cause, whether in herent or external ; and, secondly, a mental change, whereby the perception of the bodily change is accomplished. A hot substance is applied to the skin sufficient to burn ; a visible change is produced on the part to which the application has been made, shown by the in creased redness of the cutaneous surface, and the nerves of the part are so irritated that pain must be felt if the perceiving power of the mind be unimpaired. But unless the mind is conscious of the irritation excited we cannot say that a sensation has taken place. The person on whom the injury is inflicted may be comatose, or in a profound sleep, or under the influence of intoxicating or anmsthetic agents, and consequently' his perceptive powers are in abeyance. Never theless, the same physical changes take place, whatever be the state of the mind, and all the physical phenomena, which may flow from or succeed to those which are capable of excit ing sensation, may ensue upon them, and yet true sensation will not take place, unless the mind perceives and takes cognisance of the physical change induced.
It must then be regarded as a cardinal point in reference to the acceptation of the term Sensation in Physiology, that an action of the mind is necessarily involved, that act being of the nature of a recognition or per ception of the physical changes associated with the sensation.
The true organ of sensation is the organ of the mind — the brain, and especially that part of the brain which constitutes the centre of sen sation, and which extends into the spinal cord, forming the posterior horn of its grey matter. When an impression is made upon a nerve or nerves which communicate directly or indi rectly with any part of this centre, a sensation is excited, provided the intracranial portion of it be in a normal state, and provided also the connection between the cranial and spinal portions be complete and uninterrupted.
Sensations depend, as to their nature, on that of the excitant, and nerves are adapted to receive impressions from various agents, ponderable or imponderable. The mechanical qualities of bodies, heat, cold, electricity, light, sound, &c., are capable of exciting their ap propriate sensations, which the mind soon learns to appreciate and distinguish. Sensa tions thus distinguished receive the appella tion of pleasurable or of painful, according as they are agreeable or the reverse. These sensations are infinitely varied in kind and in degree. It is impossible, d priori, to deter mine how a pleasurable or a painful sensation may be excited. Nor will the experience of one person be always a guide for another, in asmuch as a sensation which may be agree able to one, may be painful or disagreeable to another.
Physiologists distinguish sensation as com mon and special: the former being that which is excited by- ordinary mechanical or chemical stimuli ; the latter is excited by special sti muli, and is exemplified in the special senses of vision, hearing, smell, taste, and touch. The nerve of vision does not, when irritated, communicate simply a feeling of pain or of pleasure ; its chief effect is to excite the sen sation of a flash of light. When the electric stream passes through the retina, a sensation is caused similar to that which the sudden presentation of a luminous object would pro duce. In like manner the mechanical or electrical stimulation of the other nerves of pure sense will create, not pain, but a feeling closely allied to that which would be excited by the application of the stimulus proper to each. This is remarkably illustrated by the effects of mechanical or electrical stimulation of the nerve of hearing and of the nerve of taste. Mechanical impulses against the tym panum occasion the sense of a dull sound, and the electric current developes a musical note. Galvanic excitation of the gustatory papillae of the tongue causes a peculiar sbur taste, and, as Dr. Baly has pointed out, the mechanical stimulation of them by a sharp tap with the fingers, occasions a taste some times acid, sometimes saline.