THE SPINAL CORD 229 



Tabes dorsalis is a disease of the spinal cord in which the 

 sensory nerves only are paralyzed, and the posterior roots 

 and Coil's columns are degenerated. Hence Coil's columns 

 are sensory tracts. 



After transverse section of the spinal cord, secondary 

 degeneration of* the fibres of Coil's columns and the direct 

 cerebellar tract above the section takes place; hence the 

 cells of these fibres lie below the section. 



Half-section of the cord. If, by injury, one lateral half 

 of the cord has been cut through, motor paralysis below 

 and on t'he same side of the injury occurs, while on the 

 opposite side there is chiefly loss of sensation. The injured 

 motor columns lie, therefore, mainly on the same side as 

 the corresponding peripheral motor nerves. The injured 

 sensory columns lie, however, chiefly on the side opposite 

 to the corresponding peripheral sensory nerves ; this depends 

 upon the above-mentioned crossing (2/;) of the sensory fibres 

 in the gray matter. 



This brief review of the sensory tracts relates, in general, the 

 facts as they are known at present ; but the conditions, in detail, 

 are much more complicated, for the long fibres in the spinal cord 

 give off branches downward and laterally, ending in the gray matter. 

 For example, each of the fibres of the posterior roots which enters 

 the posterior columns divides into two long branches, the heavier 

 one proceeding upward and finally, with Coil's column, reaches the 

 medulla oblongata ; the other going downward and, after a short 

 course, ending in the gray matter. Both these branches give off 

 collaterals which also end in the gray matter. The cells in the gray 

 substance to which the ends of the descending and collateral 

 branches go also give off neurites which, under the giving off of 

 collaterals, form long tracts or, after a short course, end in the gray 

 substance. Hence there is no such sharp distinction between the 

 long sensory tracts and the reflex tracts to be described presently, 

 as would appear from the above description. 



3. REFLEXES OF THE SPINAL CORD 



Nature of the reflex. The reflex is the transferring of a 

 stimulation from a centripetal to a centrifugal nerve through 

 the centre. This occurs involuntarily. According to the 



