326 NERVOUS SYSTEM. 



posterior part of the medulla oblongata through the entire length 

 of the spinal canal. Lodged within a cavity whose form is con- 

 tinually undergoing more or less variation, it neither fills nor fits 

 that cavity : in some places it swells in volume, in others it 

 grows contracted ; everywhere it has the appearance of having 

 shrunk from, or of being disproportionably small for, the canal 

 ill which it is contained. 



Coverings. — It is inclosed in the same membranes that envelop 

 the brain ; but, in addition to them, the fiiiperior ligament of the 

 spine serves as a covering and defence to it below. To this, and 

 to the periosteum lining the canal, its proper theca is loosely at- 

 tached by cellular, adipose, and gelatinous matter. Its dura 

 mater is derived from that which covers the brain : in being con- 

 tinued through the foramen magnum, the membrane is contracted 

 into a cylindrical sheath which loosely incases the marrow, and is 

 generally described under the denomination of theca vertebra/is. 

 About the beginning of the sacrum, the theca narrows; it re- 

 cedes from the sides of the canal, and, having inclosed the ter- 

 mination of the marrow, is extended into a point and lost upon 

 the Cauda equina. The dura mater is connected to the mem- 

 branes underneath — the membrana arachnoides and pia mater, 

 by a fine cellular tissue ; and these coverings have the same re- 

 lation to the marrow that the same membranes have to the 

 brain — of which, indeed, they may be considered prolongations. 



Structure. — The spinal marrow, stripped of its membranes, is 

 cylindroid in form, but flattened along its upper and under surfaces, 

 each of which is marked by a longitudinal linear fissure : these 

 fissures, of which the inferior is the more conspicuous one, are con- 

 tinuations of those that traverse the medulla oblongata — dividing 

 the marrow into two lateral portions, and serving as convenient 

 tracts for the spinal bloodvessels. In places, also, some trans- 

 verse fissures or little wrinkles are apparent, which are evidently 

 for the purpose of admitting of certain degrees of elongation and 

 contraction, in order to guard against extension or laceration of 

 the marrow under any motion or altered position of the spine. 

 The marrow varies in its dimensions at different parts ; and, also, 

 somewhat in its form. At its continuation from the medulla ob- 

 longata, it is large, from which to the fifth cervical vertebra it 

 gradually grows less in circumference ; here it swells again, but 

 again diminishes in entering the first dorsal vertebra. Along the 

 back it is small, and nearer approaches the form of a perfect 

 cylinder ; but in the loins it grows flatter than ever, and, a third 

 time, increases in size, until it reaches the last lumbar vertebra ; 

 from which it begins to taper, and continues so to do until it at 

 length ends in a pyramidal point, within the second piece of the 



