Below the cerebrum and lying in front of the occipital bone, we find the cerebellum, which is the seat of memory and the center for the co-ordination of muscle movements. By co-ordination of muscle movement is meant that the muscles will do just what we want them to do, that they will act harmoniously, the one with the other. The condition of Saint Vitus' Dance would be an example showing a lack of co-ordination. This part of the brain is also convoluted.
[Illustration: FIG. 15—Brain and spinal cord.]
Between the cerebrum and the cerebellum, and connecting the two, is found the pons Varolii. The word pons means bridge, and the word Varolii means to cross over. It is in this part of the brain, then, that the nerve fibers cross over to the opposite side. A person having a paralytic stroke on the right side of the body would indicate that the left side of the brain had become affected.
Joined to this is the medulla oblongata. This is the lowest part of the brain and is the connecting link between the brain and the spinal cord. The medulla controls the circulation, respiration, and deglutition (swallowing).
Closely adhering to the brain, is a delicate membrane, sinking into the convolutions, and following the surface of the brain valleys throughout. This membrane is called the pia mater. In it is found the capillaries, which supply the brain with its nutritive blood in life and with embalming fluid after death. These capillaries do not penetrate the substance of the brain, but the process is one of osmosis, absorption or transfusion. Covering the outer most part of the brain, and closely adhering to the cranial bones is a dense, tough, glistening, membrane, called the dura mater. In the dura mater is found the sinuses of the brain.
In between the pia mater and the dura mater is a delicate double membrane forming a closed sack, called the arachnoid membrane. This sac contains a serous fluid, which offers great protection to the brain. These same three membranes also cover the spinal cord, and are called all together the meninges of the brain and cord.
The brain is composed of white and gray matter. The gray matter is on the outside, and the white matter is on the inside.
_The spinal cavity_ is formed by the bones of the vertebral column. In this spinal cavity is found the spinal cord. It is cylindrical and usually about seventeen inches in length, and extends from the medulla oblongata, to the lower border of the first lumbar vertebra, where it terminates in a slender filament of gray substance.
