348 THE NERVE SYSTEM 



nerve fibers, without possessing any neurilemma. 

 They run in a vertical direction, and with their sup- 

 porting frame-work of neuroglia and connective tissue 

 the latter is derived from the pia mater and blood- 

 vessels are grouped into bundles of axones, termed 

 columns or tracts, which are marked off by fissures 

 that can be seen with the naked eye on the surface 

 of the cord. 



The Structure of the Spinal Cord. If a cross-section 

 of the spinal cord be examined, it presents a central 

 gray substance and a surrounding white substance, 

 the former consisting of bodies of nerve cells and 

 their non-medullated axones; the latter contain the 

 medullated axones arranged in columns. The neuroglia 

 pervades both the white and gray substance, and is 

 the supporting tissue frame-work for the nerve cells, 

 their dendrites, and axones. 



THE GRAY SUBSTANCE OF THE CORD. It is arranged 

 within the spinal cord in the form of two crescents 

 joined in the centre, or a figure resembling the letter H. 

 The gray substance on either side extends nearly to the 

 surface of the cord, surrounded by the white matter, 

 the posterior projections are called the dorsal or 

 posterior horn, and the anterior the ventral or anterior 

 horn; the two halves of the gray substance are con- 

 nected by a bridge of gray substance termed the 

 commissure. The latter presents in its centre a 

 narrow canal (neural) which extends the entire length 

 of the cord. It is lined by cylindric epithelial cells 

 and surrounded by a gelatinous material. 



The anterior horn of the gray substance is broader 

 than the posterior, and is completely surrounded by 

 white substance. The posterior horn is narrower and 

 approaches nearer to the surface of the cord than the 

 anterior horn does, and is enclosed by a gelatinous 

 substance called the substantia gelatinosa. In the 

 lower cervical and thoracic portions of the cord the 



