511 The hemispheres: Cortex cerebri-Corona radiata Internal capsule-Crura cerebri-Corpus callosum-The pyramidal or motor tract-The basal ganglia Corpus striatum-Optic thalamus-The cerebellum-Corpora geniculata Corpora quadrigemina. 514 Relation of cerebral development to intelligence. 515 Functions of basal ganglia; of the cerebellum-Forced movements. 519 The cortex of the brain. 520 Arteries of the brain-The Sylvian artery-The lenticulo-striate artery. 522 The functions of the brain. Hemiplegia-Crossed paralysis-Associated bilateral muscles-Yoked ' muscles and conjugate movements. 527 Cortical localisation of function. Experimental excitability of the cortex cerebri-Motor area-The visual area 537 Epilepsy Epileptogenous foci. 539 Aphasia-Motor aphasia-Sensory aphasia-Word-blindness-Word-deafness-Recovery from aphasia-Deaf-mutes. 544 The personal equation or reaction time. 546 Ratio between stimulus and sensation-Weber-Fechner law. 549 Weight-discrimination-The muscular sense. 551 Sensory judgments-Illusions Contrast theories-Helmholtz, Hering. 561 Fatigue Sleep Hypnosis. General plan of its structure.-Without entering upon any detailed description of the brain as given in anatomical textbooks, we shall in this place consider and insist upon such points only as have immediate bearings upon the knowledge of its functions. The cranium contains the two cerebral hemispheres and their basal ganglia (corpora striata, corpora quadrigemina, optic thalami), the cerebellum, and the medulla oblongata. From the point of view of physiological anatomy we regard the medulla oblongata (= spinal bulb) as being spinal rather than cerebral ; and we consider by itself the cerebellum, which, whatever its function may be, is not known to possess any action that it is customary to call 'cerebral.' As regards the basal ganglia, our attention becomes directed to them, less on account of any definite functions that have been assigned to them, than because they occupy the isthmus of the brain, and are thus on or close to the path between brain and body. Our physiological study of the brain is thus simplified, and comes to be directed almost exclusively to the surface of the hemispheres, and to the tracts that connect them with each other and with the nerve-channels emerging FIG. 273. IDEAL VERTICAL SECTION THROUGH THE CEREBRO-SPINAL AXIS. To illustrate the course of the pyramidal tract and its interlacement in the corona radiata with the commissural fibres of the corpus callosum. from the base of the brain; these are the crura cerebri and the cranial nerves. Viewed broadly, the brain is a mass of white matter, with nuclei of grey matter deeply imbedded in it, and with a sheet of grey matter, about one-fifth of a square meter in area and between 2 and 3 mm. thick, covering the folds, fissures, and convolutions of its surface. This superficial grey matter, or cortex, is the brain proper, the organ of sensation, judgment, and will; the white matter beneath leads into it sensory impulses, leads off from it motor impulses, and is simply a conducting mass composed of bundles of medullated nerve-fibres, disposed in a fanlike manner between the isthmus of the brain and the grey matter of the cortex, while below the isthmus it is continued as the white columns of the bulb and cord. This mass of white matter is known as the corona radiata; at the base of the brain, between the corpus striatum and optic thalamus, it forms a conical sheet of white matter known as the internal capsule, and proved to constitute FIG. 274.-IDEAL HORIZONTAL SECTION THROUGH THE RIGHT HEMISPHERE AND BASAL GANGLIA. To illustrate the position of the internal capsule taken in transverse section . HAL indicate the situations in the internal capsule of fibres governing the movements of the head, arm, and leg respectively. (After Charcot.) the main channel of motion and of sensation. This main channel is continued as the two crura cerebri, which traverse the pons varolii, and are continuous with the white columns of the bulb and cord. It is probable that this main stream gives and takes fibres to and from the ganglionic masses which it skirts or traverses at the base of the brain, but to what extent we cannot say, and it is important to realise that a large portion passes directly through, as a system of afferent and efferent channels, between grey matter of cortex and grey matter of cord. The clearest testimony of this is afforded by the tract of descending degeneration that has been traced from cortex through corona radiata, internal capsule, crus cerebri, pons, anterior pyramids of bulb, to the lateral column of the opposite side (the crossed pyramidal tract), and to the anterior column of the same side (the direct pyramidal tract). As regards localisation of motor and of sensory paths in the internal capsule and in the crus, pathological observations go to show that the posterior division of the internal capsule and the anterior or ventral portion of the crus (pes, crusta, or basis) are the main channels of motor and sensory impulses. It has further FIG. 275.-DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE CRUS CEREBRI AND The letters a. m. p. on the pes or crusta signify portions occupied by fibres from the anterior, middle or Rolandic, and posterior regions of the cortex. been observed that injury of the anterior two-thirds of the posterior division of the internal capsule is associated with hemiplegia ; injury of the posterior third, with hemianæsthesia; injury of the whole, with hemiplegia cum hemianæsthesia. Descending degeneration from the frontal and occipital regions of the cortex are respectively anterior and posterior to the pyramidal area in the capsule and crus, but are not traceable beyond the pons; whereas the pyramidal degeneration from the Rolandic region extends down the spinal cord (p. 486). In addition to (1) the vertical system of fibres above described, the corona radiata contains (2) a horizontal system of fibres between the two hemispheres, having their chief path through L L R Cortex L the corpus callosum, and (3) numerous but scattered association fibres between the various convolutions. These last-named fibres are difficult to trace, least so, perhaps, in the gyrus fornicatus, in which there is a well-marked longitudinal system known as the cingulum. The vertical and horizontal fibres, derived respectively from the internal capsule and the corpus callosum, are also difficult to dissociate, being closely interwoven in their expansion through the corona radiata. Capsule QTA R Crus R Pons R Bulb R Cord L L L L Relation of cerebral development to intelligence. The brain, or, more precisely speaking, the cortex of the brain, is the organ of intelligent sensation and motion. Taken in the rough, the intellectual rank of animals bears some relation to the weight of the brain. Thus the average ratio between brain-weight and bodyweight is in round numbers FIG. 276.-TO ILLUSTRATE THE But mere weight of brain is not a regular index of degree of intelligence in individual cases; there are many exceptions to the general rule. As regards man very similar considerations hold good, viz., taken in the rough, the brain-weight of distinguished men is above, while that of idiots is below, the average, and the brain-weight of civilised men is above that of savages. Yet there are many individual exceptions to the general rule. The average brainweight is: As regards sex, the brain-weight cannot be appealed to in evidence of superiority on either side. The average male European brain weighs 49 oz.; the average female European brain weighs 44 oz.; but the average body-weights in the two sexes |