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The spinal cord is an extension of the brain, extending from the base of the skull to just below the ribs. And a stem of brain tissue, with a small canal running through its entire length. The whole marrow is covered by membranes, like the brain, and is also bathed inside and out with the same protective fluid as the brain.
Like the brain, the spinal cord needs protection. While the brain is safely enclosed in a rigid skull, the spinal cord is surrounded by a set of bones called vertebrae. These form the spine, which is capable of flexing when we bend or move. At the same time, the spine must be strong enough to support the weight of the body and provide safe protection for the spine. It might seem that flexibility, strength and protection of its fragile content could not be obtained by the spine, but its ingenious construction makes it all possible.
The spine consists of more than two dozen ring-shaped vertebrae. The spinal cord passes through the hole in the center of each vertebra, and is completely protected by the bone arches. The bony protuberances of the vertebrae articulate so that each vertebra can move only slightly so as not to pinch or injure the spinal cord. Between each pair of vertebrae are small openings through which the nerves can pass, branching from the spinal cord itself. The complicated structure of the spine is held together by flexible ligament cords and powerful muscles.
The brain looks like a large, light pink walnut. Its surface is deeply wrinkled and full of folds, and its upper part is almost divided into two parts by a very deep groove. This wrinkled surface occupies most of the brain and is called the brain. In most animals the brain is quite small, but in man it has grown so large that it covers the rest of the brain.
The brain, along with other parts of the brain, grows from the brain stem, which is an expansion at the top of the spinal cord. Just below the brainstem is the cerebellum, only 1/8 the size of the brain, but quite similar in its outer appearance. It's even more wrinkled, and it's placed directly on the back of the head. O thalamus it's the hypothalamus, other smaller parts of the brain, also grow from the brain stem, being completely covered by the brain mass. A series of large spaces, or ventricles, run through the entire structure of the brain and are filled with fluid.