The central grey substance (substantia grisea centralis) consists of two symmetrical portions, one in each half of the medulla spinalis: these are joined across the middle line by a transverse commissure of gray substance, through which runs a minute canal, the central canal, just visible to the naked eye.

In a transverse section each half of the gray substance is shaped like a comma or crescent, the concavity of which is directed laterally; and these, together with the intervening gray commissure, present the appearance of the letter H.

This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from


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