Further, we find this organ in some degree duplicating the function of the second-level centres, for fibres go out from these intermediate masses to certain areas of the hemispheres, which reproduce locally the senses of hearing, sight, etc. By these fibres the functions of the senses are "projected" out to the surface of the brain, and the term "projection fibres" is applied to the nerves which make these connections. The hemispheres are not content even with the most important of all functions--the strictly intelligent--but they are jealous, so to speak, of the simple sensations which the central brain masses are capable of awaking. And in the very highest animals, probably only monkeys and man, we find that the hemispheres have gone so far with their jealousy as to usurp the function of sensation. This is seen in the singular fact that with a monkey or man the removal of the cortical centres makes the animal permanently blind or deaf, as the case may be, while in the lower animals such removal does not have this result, so long as the "second-level" organs are unimpaired. The brain paths of the functions of the second and first levels taken together constitute the so-called "voluntary circuit" (see Fig. 2). In addition to this general demarcation of functions as higher and lower--first, second, and third level--in their anatomical seat, many interesting discoveries have been made in the localization of the simpler functions in the cortex itself. The accompanying figures (Figs. 3 and 4) will show the principle centres which have been determined; and it is not necessary to dwell upon additional details which are still under discussion. The areas marked out are in general the same on both hemispheres, and that is to say that most of the centres are duplicated. The speech centres, however, are on one side only. And in certain cases the nervous fibres which connect the cortex with the body-organs cross below the brain to the opposite side of the body. This is always true in cases of muscular movement; the movements of the right side of the body are controlled by the left hemisphere, and _vice versa_. The stimulations coming in from the body to the brain generally travel on the same side, although in certain cases parallel impulses are also sent over to the other hemisphere as well. For example, the very important optic nerve, which is necessary to vision, comes from each eye separately in a large bunch of fibres, and divides at the base of the brain, so that each eye sends impulses directly to the visual centres of both hemispheres. [Illustration: FIG. 3.--Outer surface of left hemisphere of the brain (modified from Exner): _a_, fissure of Rolando; _b_, fissure of Sylvius.] [Illustration: FIG. 4.--Inner (mesial) surface of the right hemisphere of the brain (modified from Schaefer and Horsley). In both figures the shaded area is the motor zone.]