SENSATION (in physiology) may be defined to be "the perception by the mind of a change wrought in the body." According to this definition, which is borrowed from Dr. Todd, sensation involves—first, a bodily change from some cause, whether inherent or external; and secondly, a mental change, whereby the perception of the bodily cleange is accomplished. The true organ of sensation is the brain, and especially that portion of it which (to use the words of the above-named eminent physiologist) consti tutes the center of sensation, and extends into the spinal cord, forming the posterior horns of its gray matter. See SPINAL MARaow. Physiologists distinguish between common and special sensation. Common sensation exists in the skin, and in all parts of the body to which ordinary sensory nerves are distributed, and is excited by ordinary mechanical or chemical stimuli; while special sensation is exemplified iu the special senses of vision, hearing, etc. For the due action of the latter there are organs of spe• cial sensation, width, by the peculiar character of the nerves with which they are sup plied, become the recipients of impressions of a particular kind; thus, the eye is sensible to light, the ear to sound, etc. ; and if the special nerves going to these organs be irri tated, instead of pain being as in the case of an ordinary sensory nerve, there is a feeling closely allied to that which would be excited by the application of the normal stimulus, as light, sound, etc. Any ordinary sensibility those organs (the eye, ear, etc.) possess is dependent on ordinary sensory nerves, and is quite independent of the nerves of special sense.
In works on the physiology of the nervous system, we often meet with the phrases objective sensation, subjective sensation, and reflex sensation. We shall conclude this article
by a brief description of the meaning of these terms. "In the ordinary mode of exciting sensations," says Dr. Todd, " the presence of an object is necessary. This object cre ates an impression on the peripheral parts of the sensitive nerves; and the change caused by this impression being duly propagated to the center of sensation, is perceived by the mind." This, which is the ordinary form of sensation, is termed an objective sensation, in opposition to a so-called subjective sensation, in which a mental act can develop a sen sation independently of any present object. These subjective sensations are sometimes excited by the mind recalling, more or less exactly, the presence cf an object; but in many cases they are caused by physical changes in the nerves themselves, owing to an excess or deficiency of blood, or some oilier pathological causes. Thus disordered conditions of the retina or optic nerve may give rise to motes or flashes of light; disturbance of the auditory nerve occasions singing in the ears, the sound of distant bells, etc.
To understand the mode in which reflex sensations are brought about, an acquaint ance with reflex action, described in the article _NERVES AND NERVOUS SYSTEM, is requi site. As examples of this form of sensation may be mentioned the facts, that the irritation of a calculus in the bladder will give rise to pain in the thighs; that diseased liver often excites pain in the shoulder-joint; and that ice or iced drinks suddenly intro duced lute> the stomach occasion intense pain in the forehead. For further information on the subject, the reader is referred to Dr. Todd's article "Sensation," in the 4th vol. of his Cyclopredia of Anatomy anal Physiology.