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Brain and Meninges

This mass of nerve tissue, contained within the cranial cavity, is a greatly enlarged continuation of the spinal cord, and like the cord is formed by both grey and white matter. The grey matter is found on the outer surface where it forms a rather thin layer, the cortex, and in addition masses of grey matter are found embedded within the brain. The bulk of the brain is, however, formed by white matter which consists of nerve fibers. In man the brain of the adult weighs approximately three pounds. In the lower animals the brain is formed by three distinct masses, easily recognized as the forebrain, the midbrain, and the hindbrain. Because the forebrain is so markedly developed (44 oz.) in man, it folds over the other divisions of the brain in such a fashion that it is difficult to distinguish the separate portion. The forebrain consists mainly of two large cerebral hemispheres which are almost completely separated by a deep median fissure. The two hemispheres are connected, however, by large masses of fibers which carry impulses from each hemisphere to the other. The outer surface, cortex, of the cerebrum is markedly folded into convolutions or gyri, thus serving to increase greatly the total area of cortical grey matter. The midbrain may be considered as a connection between the other two divisions. The hindbrain is formed by the cerebellum, the medulla oblongata and the pons Varoli..

If a transverse section is made through the cerebral hemispheres, the outer cortex, the inner white matter and the embedded grey matter may be observed. In addition some of the cavities within the brain may be seen. These cavities, ventricles, are merely numbered, from before back, and are known as the first, second, third and fourth ventricles. The ventricles are filled with a lymphlike fluid known as the cerebrospinal fluid, it being likewise found within the tiny central canal of the spinal cord.

Surrounding the brain and spinal cord are three membranous coverings. The outer one is the heavier, tougher membrane and is called the dura mater. The middle covering is a delicate membrane resembling a spider web, hence its name, arachnoid while the inner membrane, that closely invests the brain and cord and dips into every fissure or sulcus, is the pia mater. The inner membrane, the pia, carries a rich supply of blood vessels which pass to the outer portion of the brain and cord. Inflammation of these coverings (the meninges) is known as meningitis, - hence one suffers from cerebral, spinal or cerebrospinal meningitis.

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