MENTAL PATHOLOGY. The science of abnormal mental process. The intimate depen dence of consciousness upon the functioning of the central nervous system enables us to approach the investigation of morbid mental conditions from the vantage ground of physiology. The brain, which is the substrate of mind, may, like any other organ of the body. exhibit (1) de fects—i.e. lack of some structure—or (2) ab normality of function, whether it be (a) tem porary—i.e. a disorder—or (b) permanent—i.e. a disease. `Defeetives' are, then. persons who suffer, congenitally or from early childhood, from the absence of some group or groups of mental elements in consequence of sonie underlying structural gap in the nervous system; they are the blind, the deaf, the paralytic. etc. The eases of Laura Bridgman and Ilelen Keller, the blind deaf-mutes. are typical. From careful reports of their educational progress. and from special psychological and neurological examination of their mental and physiological organization, val uable data have been secured. Temporary dis turbances of normal mental functioning :ire af forded in the consciousness of dreams :extreme inattention), hypnosis (extreme attention), and the intoxication of various drugs. Frequent at tempts have been made to examine mind as spe cific phases of it are rendered anesthetic or hypera'sthetic in these ways. Hashish (extract of Cannabis Indica), e.g., greatly magnifies our consciousness of duration and extent, and also induces visual hallucination. Chronic- mental derangement is exemplified by the various forms of insanity—mania, nadaneholia, dementia, gen eral paresis, ete.—the extended investigation of which by competent alienists has thrown much light upon the nature of the more complicated mental processes.
Abnormal mental types are, as one writer puts it, "psychological experiments made for us by Nature herself." Especially is this true when the infirmity is isolated, when a single group of mental processes—e.g. a souse department—is
either entirely lacking or extraordinarily empha sized. Such a state of affairs simplifies matters for the psychologist. Ile is able to find out the relative value of the group concerned in the nor mally organized mind, and. as a consequence, to proceed more successfully with the analysis of the adult human consciousness. (See PSYCHOL OGY.) Take, for example, eases of the aniesthesia of particular internal organs which lie beyond that experimental control which is of supreme importance in the laboratory investigation of the external sense organs. Evidence of this sort has been of weight in referring the sensation of gid diness to the semicircular canals of the internal ear. (See STATIC SENSE.) From observations of senile dementia Hughlings Jackson has estab lished the law that, in the gradual loss of mem ory with advancing age, the latest mental stuff, that acquired with most difficulty. first decays. The successive stages of dissolution consequent upon the inroads of cerebral deterioration re trace the steps of evolution. The various types of aphasia (q.v.) have been of great assistance in the solution of the problem of the cortical localization of function, as well as in the more strictly psychological problems of apperception (q.v.) and language.
BIBLIOGRAPHY. Lewis. Mental Diseases: PathBibliography. Lewis. Mental Diseases: Path- ological Aspects of Insanity (London. 1889) ; Hall, Mind, iv. (1879, 149) ; Maudsley, The Pathology of Mind (London, 1879) ; Mercier. Sanity and Insanity (London, 1S90) ; Binet, The Psychology of Ref:sot:inn, Based upon Experi mental Researches in Hypnotism ( Eng. trans.. Chicago. 1899) ; Ribot, Les maladies de la at& moire (Paris. 1891 ) Diseases of the Will ( Eng. trans., Chicago, 1894) ; Diseases of Personality (Eng. trans., Chicago, 1894) ; Sully, The Hu luau Mind, i.. 19. 74. ii., 3201. ( London, 1892) ; Titehener. An Outline of Psychology (New York, 1899).