What is known as the brain of man is really a trinity of three brains, known respectively as (1) the _medulla oblongata_, (2) the _cerebellum_, and (3) the _cerebrum_. If one wishes to limit the mental activity to conscious intellectual effort, then and then only is he correct in considering the cerebrum or large brain as "the brain." _The Medulla Oblongata._--The medulla oblongata is an enlargement of the spinal cord at the base of the brain. Its office is that of controlling the involuntary activities of the body, such as respiration, circulation, assimilation, etc. In a broad sense, its activities may be said to be of the nature of highly developed and complex reflex activities. It manifests chiefly through the sympathetic nervous system which controls the vital functions. It does not need to call on the large brain in these matters, ordinarily, and is able to perform its tasks without the plane of ordinary consciousness. _The Cerebellum._--The cerebellum, also known as "the little brain," lies just above the medulla oblongata, and just below the rear portion of the cerebrum or great brain. It combines the nature of a purely reflex center on the one hand, with that of "habit mind" on the other. In short, it fills a place between the activities of the cerebrum and the medulla oblongata, having some of the characteristics of each. It is the organ of a number of important acquired reflexes, such as walking, and many other familiar muscular movements, which have first been consciously acquired and then become habitual. The skilled skater, bicyclist, typist, or machinist depends upon the cerebellum for the ease and certainty with which he performs his movements "without thinking of them." One may be said never to have thoroughly acquired a set of muscular movements such as we have mentioned, until the cerebellum has taken over the task and relieved the cerebrum of the conscious effort. One's technique is never perfected until the cerebellum assumes control and direction of the necessary movements and the impulses are sent forth from below the plane of ordinary consciousness. _The Cerebrum._--The cerebrum, or "great brain" (which is regarded as "the brain" by the average person), is situated in the upper portion of the skull, and occupies by far the larger portion of the cavity of the skull. It is divided into two great divisions or hemispheres. The best of the modern authorities are agreed that the cerebrum has zones or areas of specialized functioning, some of which receive the sensory reports of the nerves and organs of sense, while others send forth the motor impulses which result in voluntary physical action. Many of these areas or zones have been located by science, while others remain as yet unlocated. The probability is that in time science will succeed in correctly locating the area or zone of each and every class of sensation and motor impulse. THE CORTEX.