COMMISSURAL FIBERS They connect opposite sides of the cerebrum and, like the pro jection fibers, are continuous with the radiations of Meynert. They are contained chiefly in the corpus callosum, the anterior commissure, and the commissura hippocampi; but are also found in the posterior commissure, commissura habenularum, inferior (Gudden's) and superior (Meynert's) commissures.
The corpus callosum, as already described, is the great link between the cerebral hemispheres (Figs. 33, 42 and 96). Its fibers connect both similar and dissimilar parts of the cortices; within the hemisphere, they form a prominent radiation, called the radiatio corporis callosi. Valkenburg has confirmed in man the observation of Beevor (1891) upon the marmoset, that the corpus callosum contains no fibers from the striate (visual) cor tex, though the psychic visual cortex contributes fibers to it. Valkenburg traced an abundant connection of the anterior cen tral gyrus with both central gyri of the opposite side (Brain, Vol. 36). This important contribution appears to explain the gen eral epileptic convulsion due to unilateral irritation, and the one well-known symptom of callosal lesion, viz., left-sided apraxia. J. Levy-Valensi claims that lesions of the corpus callosum in man also cause reduction of association of ideas and weakening of memory; and, sometimes, change in character and bilateral motor disturbance. The views of Levy-Valensi (Brain, Vol. 34) afford some support to the endeavor of Spitzka to establish a direct relation between the size of the corpus callosum and the mental power of the individual. It is the corpus callosum, chiefly, that makes it possible for the two hemispheres of the cerebrum to act together as one organ. Philogenetically, it is of recent development, since it is not found below mammals. It
is developed in the lamina terminalis just above the anterior neuropore; the anterior commissure is thrown across just below the neuropore (Johnston).
The anterior commissure (Figs. 33, 5o and 96) joins the opposite temporal and occipital lobes together (pars occipilo temporalis), the limbic lobes with the contra-lateral olfactory tracts, and the olfactory tracts with each other (pars olfactoria).
It is supplementary to the corpus callosum and associates re gions not joined by the great commissure, especially the cortex of the tentorial areas of the cerebral hemispheres. In size it va ries inversely as the corpus callosum. Its importance diminishes with the appearance of the corpus callosum in the lower mam malia and it continues to decrease as the higher forms are approached. Below mammals it is said to be the most impor tant connecting link between the hemispheres and is philogen etically very old (see p. 133).
The commissura hippocampi, the lyre (Fig. 47), unites the hippocampal gyrus, dentate fascia, and the hippocampus with their fellows of the opposite side. This is the commissure of the pyriform lobes, the cortical areas of smell.
The lamina terminalis becomes thickened by the invasion of gray substance due to the fusion of the medial olfactory nuclei. That thickening Elliot Smith designated the "precommissural body." The precommissural body forms the anterior wall and a part of the floor of the median ventricle of the telencephalon, the aula. According to Johnston it pushes up into the supra neuroporic lamina and constitutes the bed for corpus callosum, commissura hippocampi and corpus fornicis. The part of the precommissural body inclosed by the three structures just named forms the septum pellucidum.