THE SPINAL CORD.—GENERAL. 481 
stance. The cord may be regarded either as an instrument for 
the reception and generation of impulses independent of the 
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_¥ ie. 342.,—Transverse section of spinal cord of child six months old, at middle of lumbar 
region, showing especially the fibers of gray substance. 1 x 20. (After Gerlach.) a, ante- 
rior columns ; 0, posterior columns; c, lateral columns; d, anterior roots; e, posterior 
roots; f, anterior white commissure; g, central canal lined by epithelial cells; h, con- 
nective-tissue substance surrounding it ; 7, transverse fibers of gray commissure in front, 
and k, the same behind central canal; 1, two veins cut across; m, anterior cornua ; 7, 
great lateral cell group of anterior cornua ; o, lesser anterior cell group (column); p, 
smallest median cell group; q, posterior cornua; 7r, ascending fasciculi in posterior 
cornua ; s, substantia gelatinosa. 
brain; or as a conductor of afferent and efferent impulses des- 
tined for the brain or originating in that organ. As a matter 
of fact, however, it is better to bear in mind that the cord and 
brain constitute one organ or chain of organs, which, as we 
have learned from our studies in development, are differentia- 
tions of one common track, originating from the epiblast. 
While the brain and the cord may act independently to a 
very large extent, as may be shown by experiment, yet it can 
not be too well borne in mind that in the actual normal life of 
an animal such purely independent behavior must be exceed- 
ingly rare. We are constantly in danger, in studying a sub- 
ject, of making in our minds isolations which do not exist in 
nature. When one accidentally sits upon a sharp object, he 
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