. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . emispheresseparated by the great longi-tudinal fissure. of Sylvius^ above the ear, and the fissure of Rolando^ extending from the crownobliquely forward and down-ward. The General Structure of theBrain.—A section across thebrain shoves it to be composedof a thin outer layer of graymatter^ called the cortex^ andnear the center other massesof gray matter, while tlie re-mainder appears white. Ex-amination wntli the microscopeshows that the gray matter iscomposed of ve
. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . emispheresseparated by the great longi-tudinal fissure. of Sylvius^ above the ear, and the fissure of Rolando^ extending from the crownobliquely forward and down-ward. The General Structure of theBrain.—A section across thebrain shoves it to be composedof a thin outer layer of graymatter^ called the cortex^ andnear the center other massesof gray matter, while tlie re-mainder appears white. Ex-amination wntli the microscopeshows that the gray matter iscomposed of very irregular,spherical, or oval cell bodies from which extend threadlike processes, called fibers^ forming the white r) cell body with all its processes is a true nerve cell, or neuron. There are usually several short processes, and frequently one long process, the axon., called by the older books axis cylinder process (Fig. 148). Some of the axones are more than two feet long and extend from the brain to near the end of the spinal cord. Axones are commonly called nerve fibers, Those which cross from one side of. Fig. 147. —Cross section of thebrain, c, band of fibers calledcorpus callosum. The thinmembrane around the outsideis the dura mater. Note thedark layer of gray matter orcortex on the surface. THE STRUCTURE OF THE SPINAL CORD 219 the brain to the other and connect parts of the same nameform bands, known as commissures^ like the pons largest commissure is the corpus callosum, uniting thehemispheres of the cerebrum. Other fibers connect onelobe of the cerebrum with another. The Spinal Cord. — The spinal cord is securely fastenedin the upper three fourths ofthe canal of the is about as thick as thelittle finger, and is surroundedby other membranes like thoseof the brain. It is round,except in the shoulder andlumbar region, where it isslightly flattened. In thesetwo places it is also enlarged,owing to the great number ofnerve fibers
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