Of course, in order to do this, the hemispheres have to grow considerably, and thus we find them enveloping the rest of the fore-brain and swamping it in structure as well as in function. Diagram 62 indicates how this is done, while Diagram 63 shows roughly the proportion and position the different parts of the brain ultimately attain. Finally, Diagram 64, which is rather more realistic, but still much simplified, presents a view of the organ in the head. The size of the cerebral hemispheres, compared with the rest of the brain, is especially remarkable. So, too, is their endeavour to increase their surface still more by throwing it into deep folds. (See Diagram 64.) These two features vary with the position of the animal in the scale of development; in man, who stands highest in intelligence and dexterity, the hemispheres are very large indeed compared with the other organs, and seamed all over with a maze of winding furrows. Another remarkable feature is the extreme degree to which specialization is carried out. Different parts of the body are represented, each by a small area of the cortex, or surface layer, and we know at what spot on the cortex such sensations as sight and hearing are perceived, and from exactly what little patch the impulse to move each limb emanates. In the accompanying diagrams (65 and 66) these areas are mapped out, their locality being fixed by the principal folds which act as landmarks on the surface of the hemisphere. [Illustration: DIAGRAM 63.—RELATION OF DIFFERENT PARTS OF THE BRAIN.] There is another important fact which we must not omit to mention in speaking of this localization: each hemisphere presides over the opposite side of the body. Early in development the nerve fibres from the eye cross over to the opposite side of the brain, and the afferent fibres from the lower parts of the body have accordingly to follow suit. Then, as the efferent fibres—_i.e._, those which set the muscles in motion—have to bring about the movements in response to information received, they must also cross to get back to the side from which it came. So, if a tumour grows inside the head on the right side, it is the left eye which becomes sightless, or the left hand which grows numb and powerless, according to the part of the cortex which is pressed upon.