Q22 PHYSIOLOGY. 



cuneiform columns. They are the root-zones of Lissauer. The one 

 is internal, the other external. The two zones are formed by the 

 posterior root-fibers at their entrance into the cord. They have the 

 same properties as the posterior roots and undergo ascending degen- 

 eration under the same conditions that produce it in the latter. 



Degeneration. 



Descending Degeneration. The crossed pyramidal, the direct 

 pyramidal, the vestibulo-spinal, the comma tract. 



Ascending Degeneration. Coil's, Burdach's, Gowers' (ascending 

 antero-lateral cerebellar), direct cerebellar, Lissauer's tract. 



Roots of Nerves. 



The spinal nerves, thirty-one pairs in number, exist throughout 

 the entire length of the cord. 



The anterior root-fibers are composed of large nerve-tubes which 

 lose themselves, for the most part, in the ganglionic cells of the 

 anterior horns of the same or opposite sides. 



The posterior root-filers are composed of fine tubes. After hav- 

 ing arisen in the intervertebral ganglia they go toward the postero- 

 lateral groove, where they enter the cord. There are here two groups 

 of fibers: one external, the other internal. 



The external root-fibers penetrate into the gelatinous substance 

 of Eolando, where they become ascending. After a more or less 

 lengthy course they pass into the ganglionic cells of the posterior 

 horn. 



^The internal root-fibers, which pass into the posterior column, 

 become lost either in the cells of the posterior horn or in the vesi- 

 cular column of Clarke. Some very long fibers ascend to the nuclei 

 of Goll and Burdach in the medulla, where they terminate. 



Some of the fibers traverse the posterior commissure to pass 

 either into the anterior horn of the opposite side (and act in reflex 

 motor actions) or into the posterior horn or descend in the cord as 

 fibers of the comma tract. 



Commissures of the Cord. 



The wliite, anterior commissure is formed by a body of fibers 

 which decussate upon the median line to pass into the lateral half 

 of the cord opposite to that from which they came. It forms the 

 major portion of the fibers of the direct pyramidal tract. This tract 



