The position of the tentorium can easily be ascertained in your own head by the fact that where it crosses the median line there is a little projection of bone called the occipital knob, very prominent on some persons, barely perceptible on others. After locating the occipital knob, a horizontal line forward will give us the portion of the tentorium. When we carry this line forward just over the cavity of the ear, thus locating the tentorium, we easily recognize below it the rounded prominence on each side in which the two hemispheres or halves of the cerebellum lie, with a depression between them on the median line. To make these and other observations on the head (which no one should neglect), the hand should be placed firmly on the scalp, so that as it slides on the bone we feel the form of the skull beneath. In most persons a distinct depression will be felt along the line of the tentorium, separating the cerebrum and cerebellum--the cerebellum being located at the summit of the neck, and extending down about as low as the end of the mastoid process, which is the large, long prominence just behind the cavity of the ear. The cerebellum may be regarded as the physiological and the cerebrum as the psychic brain, for the cerebellum is void of intelligence and volition, but has important influences on the body. It may be considered, like the spinal cord, an intermediate structure between the controlling and conscious brain and the corporeal organs. The tentorium does not entirely separate it from the cerebrum, for anteriorly it is open to permit the passage of the fibres which connect the cerebrum with the spinal cord and the cerebellum,--fibres which pass up midway between the right and left ear, so that a bullet fired horizontally through from ear to ear would sever the connection of the cerebrum with the bodily organs, producing instant death. This will be understood by looking at the profile of the interior of the right hemisphere, on which we see the position of the pons and the medulla and their relation to the cerebrum by their ascending fibres. As these ascending fibres correspond to a position just above the cavity of the ear, and as they are the channels of all muscular impulses, the reader will perceive that breadth of head immediately above the cavity of the ear must be associated with muscular impulsiveness. The position of the cerebrum in the cranium may be best understood by sawing the head in two horizontally, taking out the brain, and looking down into the base of the skull, in which we see anteriorly a shelf for the front lobes, behind which are the cavities for the middle lobes, and behind that the rounded cavities for the cerebellum. [Illustration]