CEREBRAL LOCALIZATION.
The various subdivisions of the brain are broadly divided, with regard to their functions, into two chief groups, the higher and the lower. The higher subdivisions are the cerebral hemispheres, and in these the cerebral cortex, which through the great development of the cerebral mantle and the formation of the convolutions attains such extraordinary expansion, plays the principal role and represents the material substratum of intellectual activity. The lower subdivisions are interposed between the cerebral hemispheres and the spinal cord and include the medulla oblongata, the pons, the cere bellum, the region of the corpora quadrigemina, and the cerebral ganglia—that region, therefore, also designated as brain-stem. These lower parts possess no direct import for intellectual activity, but have rather the task of regulating, independently of conscious ness or volition, the many functions necessary for the maintenance of the body. " The lower brain segments supply an apparatus, by which the general condition of the body may be reflected from within. For the moulding of the intellectual processes, the mechanism of the cerebrum proper is authoritative."—(Flechsig.) It is unquestionably the service of the anatomist, Franz Joseph Gall, first to have recognized the significance of the cerebral cortex for intellectual activity. Since Gall, anatomists have ceased to seek a definite point in the brain, to which all motor and sensory nerves converge and which might be identified anatomically as the seat of the centralized soul. As well known, Rene Descartes interpreted the pineal body as the organ of the soul ; Sbmmering located the sensorium commune in the fluid of the ven tricles ; according to Varolius, the soul had its seat in the soft brain-substance ; Thomas Willis regarded the central ganglia as perception-centres, and the corpus callosum as the seat of the imagination, while he placed thought within the cerebral convolutions. Gall, moreover, established the principle, that the individual convolutions are not all intellec tually equivalent and in this fundamental view already approached the modern theory of localization. In setting up his own localization theory he went too far, in that he sub divided the entire cerebral surface into twenty-seven separate areas, which areas were the carriers of definite intellectual faculties and, further, that along with the greater develop ment of such a definite brain-area, a corresponding stronger projection appeared on the skull. Following the theory, the possibility was assumed, that, by careful examination of the skull, a person's endowment or character might be determined. In the scientific world, Gall's phrenology did not long endure. Although the present theory of localiza
tion differs entirely in its essentials from phrenology, we must nevertheless admit that localization was advanced more by Gall than by the labors and views of the physiologist. Flourens, who defended the theory of the equivalence of the parts of the cerebrum. Gall and his pupil, Bouillaud (1825), had already learned that circumscribed injury of the cerebrum in the frontal region may lead to disturbances of speech.
According to Gall and Bouillaud, in 1836 the French physician, Marc Dax, furnished the proof, that motor aphasia appeared only after disease of the left cerebral hemisphere, and in 1861 Broca announced the theorem, that particularly the left third frontal convo lution was the seat of speech ; hence this region is even to-day commonly called Broca's convolution.
This discovery of the motor speech-centre by Broca was the foundation of the theory of localization. Although the proof of the functional variation of the cerebral cortex shattered Flourens' teaching of functional equivalence, this theory was finally entirely overthrown, not only by further pathological experience, but especially by experimental physiology, since in 1870 Fritsch and Witzig discovered the electrical irritability of the cerebral cortex. These investigators succeeded in inducing movements of certain parts of the body by stimulation of definite cortical regions by means of the galvanic current, and, further, reached the conclusion, that while stimulation of certain cortical regions produced movements, no such result followed stimulation of other regions. These investigations were. confirmed and supplemented in 1873 by those of Ferrier, who employed the faradic current instead of the galvanic. In consequence of these observations, it was possible to establish a definite cortical region as the centre for movement. Other investi gators, particularly Nothnagel, Carville and Duret, Goltz and Munk, subsequently succeeded, reversing the order, in producing paralysis of certain muscles and impairment of certain sensory activities by removal or destruction of definite cortical regions. These labors, along with the numerous investigations of other workers, have established with increasing stability the localization of the functions of the cerebral cortex.
It is admitted, therefore, that the individual regions of the cerebral surface are not equivalent, but of entirely different significance. Each cortical field presiding over a definite function is designated as a centre, and of such cerebral cortical centres, although as yet not accurately delimited, we recognize the following.