In order that the brain cells may receive and store up memories, it is evidently necessary that nerve paths from the sense organs should lead into the brain; so, if we go back to the sensory nerve cells, we shall find that in nearly every one of them, if not in every one, one of the branches makes a nerve junction with a connecting cell, which either extends directly into the brain, or links with other connecting cells which do. This is true not only of the distance sense organs, but also of the contact sense organs and to some extent of those inside the body. The path by which the brain is reached from the sense organs that are located in the body is over the sensory nerve cells to the spinal cord, and along the spinal cord by way of connecting nerve cells to the brain. All the distance sense organs are located in the head, so their nerve paths lead into the brain directly over cranial nerves; for the eye, the optic nerve; for the ear, the auditory nerve; and for the organ of smell, the olfactory nerve. One very interesting fact about the connection of the sense organs with the brain is that the nerve paths in every case cross from the left-hand [Illustration: DIAGRAM OF NERVE PATHWAYS _B_, brain; _S. C._, spinal cord] side to the right-hand side, and vice versa, so that all the sense organs in the left half of the body have their connection with the right half of the brain, and those in the right half of the body with the left half of the brain. We know of no reason why this should be so. It is merely an interesting fact. A diagram showing the path from a sense organ in the body and from one of the head senses to the brain is given in the accompanying figure. [Illustration: DIAGRAM ILLUSTRATING THE GENERAL RELATIONSHIPS OF THE PARTS OF THE BRAIN _A_, cerebrum; _B_, cerebellum; _b_, _C_, _D_, brain stem. (From Martin’s “Human Body”)] The brain in man and in all the higher animals is a very complicated organ made up of a number of different parts. To simplify the description as much as possible we shall omit all account of smaller subdivisions and speak only of the most important parts. These are three in number, named the _cerebrum_, the _cerebellum_, and the _brain stem_. Their location with reference to one another is shown herewith.