sight is at the back of the cerebrum down at its lower margin; the arrival platform for the senses of touch, pain, and the like are just about at the top of the brain. The arrival platform for the sense of hearing is down at the side. The location of these arrival platforms has been worked out by studying the effects of brain diseases. It has been found, for example, that persons who are blind, although there is nothing the matter with their eyes, are so because of a disease of the lower back part of the cerebrum, and persons who are deaf, even though their ears are perfect, are so because of diseases in the parts of the cerebrum at the sides. The only way to explain these findings is by supposing that the arrival platforms of these senses are located in the parts that are found diseased. The cells which stop the nervous disturbances coming in from the sense organs and so serve as the seat of memories of sensations are in the arrival platforms, so that when we recall how something looked, for example, the nerve cells which are active in this recollection are those at the lower back part of the cerebrum. All these arrival platforms taken together occupy only a very small fraction of the whole of the cerebrum, so that evidently it has more to do than simply to register these memories. One additional thing we shall look for in the cerebrum is a pathway from the brain to the muscles whereby the memories that are stored in the brain can make themselves effective in arousing the muscles to activity. There is such a pathway, and it is one of the best marked of all the nerve pathways of the body. It is made up of nerve cells which start just about at the top of the cerebrum, a little in front of the middle and pass down into the brain stem; these cross over from the side in which they started to the opposite side; and go on down the spinal cord to make connection by nerve junctions directly with the motor nerve cells. Thus from the brain to the muscles the nerve pathway is made up of just two nerve cells. Since this pathway crosses over from one side to the other in the brain stem, the whole right half of the body, both sensory and motor, connects with the left half of the brain, and vice versa. The part of the cerebrum in which this nerve pathway starts is called the _motor area_ of the brain. We have now seen how nervous disturbances from the various sense organs can come into the brain and be registered there as memories. We have also seen that there is a nerve pathway by which nervous disturbances starting in the motor area can pass out to the muscles, and arouse them to activity. We have left to see how the connections are made between the incoming and the outgoing disturbances, or in [Illustration: DIAGRAM OF MOTOR PATHWAY _B_, brain; _S. C._, spinal cord; _M_, muscle]