216 



THE NERVOUS SYSTEM. 



in the posterior horn are smaller and less numerous nerve- 

 cells ; (c) the vesicular column of Clarke is a column of medium- 

 sized nerve-cells, not continuous along the entire cord, on the 

 inner sides of the posterior cornua ; (d) the intermedio-lateral 

 column is a series of small nerve-cells laterally situated (in the 

 " lateral horn ") in the dorsal portion of the cord. 



FIG. 84. 



Cross-section of human spinal cord, middle dorsal region (Rchafer). a, 5 c, nerve- 

 cells of anterior horn ; d, intermedio-lateral column of nerve-cells ; /, nerve- 

 cells of Clarke's column ; g, nerve-cells of posterior horn ; cc, central canal ; ac, 

 anterior white commissure. 



The central canal is a small tube in the middle of the gray 

 commissure extending the whole length of the cord and con- 

 tinuous with the ventricles of the brain ; it is lined with a 

 single layer of columnar epithelium-cells, ciliated in youth, 

 surrounded by neuroglia. 



The neuroglia capping the posterior horns and surrounding 

 the central canal the " substantia gelatinosa " is somewhat 

 homogeneous and glistening. 



Ganglia. 



Ganglia are small macroscopic to microscopic aggregations 

 of nerve cells and fibres, occurring abundantly in various 

 parts of the body. Since they contain nerve-cells, their 



