448 



TEXT-BOOK OF PHYSIOLOGY. 





has been differentiated into a number of specialized tracts which 

 have different origins, destinations, and functions. They may be 

 divided, however, into efferent, afferent, and associative fibers. (Fig. 

 206.) 



i. The anterior column, comprising that portion between the 

 anterior longitudinal fissure and the anterior roots, has been sub- 

 divided into: 



(a) The direct pyramidal tract, or column of Turck. This tract 



borders the longitudinal fissure and extends from the upper extremity 



.of the cord as far down as the mid-thoracic region. From above 



downward this tract diminishes in size, for the reason that its fibers 





Column of Lissauer. 



FIG. 206. TRANSECTION or THE CERVICAL SPINAL CORD SHOWING ITS CHIEF SUB- 

 DIVISIONS. (From Mills' "Diseases of the Nervous System.") 



or their collaterals cross at successive levels to the opposite side of the 

 cord by way of the anterior commissure to enter the gray matter of 

 the anterior horn. The cells of these axons are located in the cortex of 

 the cerebral hemisphere of the same side. The terminal filaments of 

 these fibers or axons are in physiologic relation with the dendrites of 

 the cornual cells. When divided in any part of their course, these 

 fibers undergo descending degeneration. They are therefore efferent 

 neurons and of the second order. 



(ft) The anterior root zone. This tract lies external to the pyram- 

 idal tract, surrounds the anterior horn of the gray matter and ex- 

 tends throughout the length of the cord. It is composed of short com- 



