In one case the first occipital convolution sank nearly beneath the surface, the next occipital gyrus projecting over it, forming a parietal operculum. There also seems to be a strong tendency to form annectant gyri in the upper part of the parieto-occipital fissure. In no less than six hemispheres of 15 brains were these supplementary gyri found more or less complete. In one case on both sides, in five cases on one side, the parieto-occipital fissure cut through the first occipital convolution into the interparietal fissure. A tendency of the transverse occipital fissure to approach the parieto-occipital fissure is very apparent, though in no case do they coincide. The folds of the cerebral cortex, from a lack of the stimulus of healthy growth, sometimes revert to forms resembling those found in other groups of the animal kingdom. The fundamental factors of thought and action, as Spitzka terms them, are two: perceptions and motor innervation. These are, in other words, the units of thought and action. They can be properly referred to nerve cell groups as their anatomical seat, and, as far as intellect is in question, to the cell group represented in the more or less diffused and dovetailing areas of specialised function in the cortex cerebri (Fig. 116). But the largest hemisphere known, with the most crowded and most highly developed nerve cells, and the most extensive connections with the periphery, and the most perfect projection of that periphery in its intricately convoluted mass would, functionally speaking, represent nothing but a mass of pigeon-holed impressions stored away without method and without purpose, useless to the organism were it not for those arched fibres uniting the different cortical centres with each other. [Illustration: FIG. 116.]