Page:Mind (Old Series) Volume 9.djvu/111

 BILATERAL ASYMMETRY OF FUNCTION. 99 So great is the asymmetry of the gyri, as well as the cerebral functions, that Exner was obliged to make out separate tables for each hemisphere in his extended study of psychophysical locali- sation from hospital records. A notable case of this sort was observed by Dr. D wight in the brain of Chauncey Wright. Various kinds of asymmetry have been noted in the caudate nucleus, the olivary bodies, and especially in the fissure of Eolando, and in the arterial system of the hemispheres. It has been stated, and also denied, that the left frontal convolution develops first ; it has also been said that embryonic features are more likely to persist in the right. If there is a defect of develop- ment in the right hemisphere, as is more common, or if there is lesion there, the left portion of the cerebellum, connexion with which may be traced along the lines of tract-development and secondary generation, is likely to be affected. The homologous hemispheres are connected by the commissures and decussions, the functions of which are rarely affected by disease, and which have been but little investigated, and very probably have more conductive power and less resistance than even the fibres of the projection-system. According to Meynert, the basis or pes cruris, arising chiefly from the corpus striatum and the lenticular nucleus, crosses in the medulla to the opposite lateral column of the spinal cord, and this is the way over which voluntary impulses pass from the brain to the muscles, while the fibres of the tegmentum, arising from the thalainus and corpora quadrigeuiina and mediating reflex movements, do not decussate. Flechsig, who derives a large part of the fibres of the basis directly from the cortex through the internal capsule, finds their decussation in the pyramid unsymmetrical, more crossing from the left brain to the right of the cord than in the opposite direction. Sometimes nearly all, and in rare cases almost none, of the pyramidal fibres decussate, great individual variation being observed. The chief transverse commissure, the corpus callosurn, first makes its appearance in placental mammals, and is, according to Huxley, " the greatest and most sudden modification exhibited by the brain in the whole vertebrate series ". Of its functions, little is specifically known. The few recorded cases of its congenital defect or absence have been marked by great bodily and mental weakness. Brain-diseases are usually asymmetrical, and certain of their most marked forms, hemiplegias, encephalitic processes, yellow spots, &c., &c., are most commonly seated in the left hemi- sphere. Deviation from bilateral symmetry in skull or brain, though in extreme cases always attended by mental defect, may be considerable without morbid symptoms. Persistent and mor- bid sense-illusions are often unilateral ; in melancholia the two sides of the face often differ noticeably in motor innervation. Aphasia in fourteen out of fifteen cases, according to one estimate, is a disease of the left brain. In this disease the language of ideas may be lost, while the primitive, more emotionaJ bilateral