272.  The Spinal Cord.  This is a long, rod-like mass of white nerve fibers, surrounding a central mass of gray matter.  It is a continuation of the medulla oblongata, and is lodged in the canal of the spinal column.  It extends from the base of the skull to the lower border of the first lumbar vertebra, where it narrows off to a slender filament of gray substance.

The spinal cord is from 16 to 18 inches long, and has about the thickness of one’s little finger, weighing about 1-1/2 ounces.  Like the brain, it is enclosed in three membranes, which in fact are the continuation of those within the skull.  They protect the delicate cord, and convey vessels for its nourishment.  The space between the two inner membranes contains a small quantity of fluid, supporting the cord, as it were in a water-bath.  It is thus guarded against shocks.

The cord is suspended and kept in position in the canal by delicate ligaments at regular intervals between the inner and outer membranes.  Finally, between the canal, enclosed by its three membranes, and the bony walls of the spinal canal, there is considerable fatty tissue, a sort of packing material, imbedded in which are some large blood-vessels.

273.  Structure of the Spinal Cord.  The arrangement of the parts of the spinal cord is best understood by a transverse section.  Two fissures, one behind, the other in front, penetrate deeply into the cord, very nearly dividing it into lateral halves.  In the middle of the isthmus which joins the two halves, is a very minute opening, the central canal of the cord.  This tiny channel, just visible to the naked eye, is connected with one of the openings of the medulla oblongata, and extends, as do the anterior and posterior fissures, the entire length of the cord.

The spinal cord, like the brain, consists of gray and white matter, but the arrangement differs.  In the brain the white matter is within, and the gray matter is on the surface.  In the cord the gray matter is arranged in two half-moon-shaped masses, the backs of which are connected at the central part.  The white matter, consisting mainly of fibers, running for the most part in the direction of the length of the cord, is outside of and surrounds the gray crescents.  Thus each half or side of the cord has its own gray crescent, the horns of which point one forwards and the other backwards, called respectively the anterior and posterior cornua or horns.

It will also be seen that the white substance itself, in each half of the cord, is divided by the horns of the gray matter and by fibers passing from them into three parts, which are known as the anterior, posterior, and lateral columns.