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TEXT-BOOK OF PHYSIOLOGY. 



specialized areas in the cerebral cortex in which these different sensations 

 arise. 



Structure of the White Matter. A transverse section of the cord 

 shows that the white matter completely covers the gray matter except where 

 the posterior horns reach the surface. Anteriorly the white matter of each 

 lateral half is connected by a narrow strip or bridge of white matter, the 

 anterior commissure. Microscopic examination shows that the white matter 

 is composed of vertically disposed medullated nerve-fibers which are devoid 

 of a neurilemma. These fibers are supported partly by a framework of 

 connective tissue, and partly by neuroglia. The white matter of each side 

 of the cord is anatomically divided into an anterior, a lateral, and a posterior 

 column by the anterior and posterior roots of the spinal nerves. 



FIG. 232. TRANSECTION OF THE CERVICAL SPINAL CORD SHOWING ITS CHIEF SUBDIVISIONS. 



(After Mills.) 



Classification of the Nerve-fibers. From a study of the embryologic 

 development of the white matter and of the degenerative changes which follow 

 its pathologic and experimental destruction, it has been differentiated into 

 a number of specialized tracts which have different origins, destinations, 

 and functions. Some of the more important tracts are shown in Fig. 232. 

 They may be divided, however, into efferent, afferent, and associative fibers. 



i. The anterior column, comprising that portion between the anterior 

 longitudinal fissure and the anterior roots, has been subdivided into: 



(a) The direct pyramidal tract, or column of Tiirck. 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 or their collaterals cross at 

 successive levels to the opposite side of the cord by way of the anterior com- 

 missure to enter the gray matter of the anterior horn. These fibers are the 

 continuations of fibers which take their origin in cells which are located in 



