Page:The New International Encyclopædia 1st ed. v. 17.djvu/865

SENSATION. tive differences bctwoeu a blue niiil a toiio. o. g., must necessarily be eunibrous and analogical; whereas the mention of eye and ear is >liurt and adequate. Psyeliology tlicrefore follows the linie- lionored custom of referring sensation-systems, modalities or senses to the organs of sense. Sensation in physiology is the perception by the mind of ehange wrought in the body. It is by means of sensation that the mind ol)tains a knowledge of the existence both of the dillVrent parts of the body and of the external world. The brain is the true organ of sensation, but besides this there must be pereejitive organs for receiving and conducting tissues (nerves) for conveying impressions to the sensorium. Sensa- tions are usually classed as cotiniuni ami special. Under the former head are included all sensa- tions that cannot be localized in any particular part of the boily, such as fatigue, discomfort, faintness, satii'ty. Inmgcr, and thirst. In this eUiss are also included irritations of the nuieous membranes, of the respiratory tract that excite cough; the desire to defecate or urinate, and. in females, the sensations that precede parturition; and itching, tingling, burning, and aching. The muscular sense, by which muscular ellurts are perceived and regulated, nuist aUo be considered as a common sensation. KiHcial soisrilioiis are five in number: touch, taste, smell, hearing, and sight. An important distinction between com- mon and special sensations is that by the former certain changes in various portions of the body are perceived, wliile from the latter knowledge of the external world is gained in addition. It is to be remembered that the seat of sensation lies in the brain and not in the special organs, although it is connuonly said that we hear with the ear, see with the eye, etc., whereas in reality these organs merely receive impressions. Objcctire seiifiatioiis are those excited by some object in the outside world; siibjcctirc sciiKOtioiis originate within the brain itself. Through habit the mind is accustomed to connect all sensations with external causes, and thisi dilliculty of sepa- rating objective and subjective sensations often gives rise to illusions. These may be aural. o)iti- cal, or tactile, and are strikingly exemplified in the various forms of delirimn. Certain disorders of sensation afTeet the nerves both of conunon and special sensation. These may be roughly classified as hypera'sthesia. anaesthesia, and pariesthesia. llyiiera'sthesia is an increased sensibility to painful impressions. It is seen in its most severe form in gunshot wounds of the nerves, and is a constant accom- paniment of neuritis. Ana^sthesiir is a loss of sensibility complete or partial, and is produced by contact with various drugs (see An.e.stuet- ICS), exposure to cold, and certain disorders of the nervous system. Paroesthcsia is a manifes- tation of disturlied sensation characterized by a number of subjective sensations such as nundj- ness. prickling, tingling, and burning. It may aflfect any part of the body surface, and occurs in a wide variety of nervous diseases. See Xer- vous System .vnd Brain. Consult: .Tames, Principles of Psiichologi/ (New York. ISnO) ; Wundt, Pliiisiolofiischc Psi/- cholofiic (Leipzig. 1S0.3) : Ladd. Psiicliolofiu. De- scriptive and Explanntnrti (New York. 1804) ; Kuelpe, Outlines of Psi/cholo!i. trans. (London, 1895) ; Titchener, Outline of Psychology (New York, I.Si)0) ; id., Experimental Psychology (ib., I'.tOI). SENSATIONALISM (sometimes called Sk.nni Ai.i.sM ). A ti rill used to deHiginite the theory that the total content of consciousness in of sense origin; that all the higher activities of mind, such as judgment and reasoning, are the results left by the impressions originally made upon the tiiljiilii riisd of the niiiul by external oli jccts. These impressions, at first imconnected, are supposed to have entered into nnituul relation by virtue of the laws of association I see ..s.sO- ciATlo.v OF luEASj. . g sensationalists are to be mentioned the Sophists (i|.v.) of antii|uity, and Hume (q.v. ) and Coiidillae ic|.v.) and their lollowers in modern times. Locke is a sensa- tionalist with large infusion of rationalism (ipv. ) in his doctrines. The classic c-xpression of the principle of sensationalism is given in the Latin sentence, Mhil isl in inlteii (ISll). Two si>ti'rs, Klinor and -Marianne Dashwood, illustrate these two cpiali- ties, the course of the story showing the elleets of sutTering on the impulsive, uncontrolled nature of one and on the sedate, unselfish disposition of the other. The too evident purpose haniiier:) the story, which contains some excellent cliar- acterizations, as Mrs. Dashwood, her selfish son, the commonphue Middletons, and vulgar but kind Mrs, .Ii'niiings. SENSE ORGANS. See Nervous System, EVOH TlOX OF THE. SENSES, Sexsiiulity. See Sensation. SENSITIVE BRIER. See Sensitive Plant. SENSITIVE PLANT. A common name of certain species of Mimosa, so called on account of the irritaliility (q.v.) of their leaves. Those species which are most irritable are herbaceous or half-shrubby i)lants with beautifully divided ]iinnate leaves. The leaHets close upward in pairs when touched, and on repeated or rough touching the leaflets of the neighboring pinnie also close together, become depressed, and lastly the whole leaf hangs as if withered. If the stem is shaken, all the leaves exhibit the same phenomena, .fter a short time the leafstalk rises, and the leallet.s expand again. On account of this curious and interesting proiicrty, some of the sensitive plants are frecpicntly cultivated in hothouses. The same faculty is possessed by the sensitive brier (Schrankia), two or three species of which are indigenous to the Southern United Stales, and also by the stamens and styles of many plants, especially of certain cacti. By extension, all plants which respond to contact stimuli are said to be sensitive, and in the widest sen-c all plants may be included. Some plants exceed in sensi- tiveness the sense organs of the human boily. SENSITIVITY (from scnsilire, from OF,, Vt. sensilif. from Lat. scniire. to perceive). . term used in ])sycbophysics. meaning "the bare capacity of receiving and eonimnnicating sensa- tions.' It is subdivided into modal sensitivity (having reference to a whole sense departiuent) and sensibilily (having reference to individual sensations). Modal sensitivity is measured by the number of sensations possible to a given sense, e.g. the ear's modal sensitivity is given by 11,000. the number of distinguishable tone quali-