596 NERVOUS SYSTEM. 



the sensory conductors run in the columns of Goll (Flechsig). The columns 

 of Goll, however, exist only in the cervical and dorsal regions. 



The sensory conductors do not decussate at any particular point as do the 

 motor conductors in the crossed pyramidal tracts. The fibres from the pos- 

 terior roots of the spinal nerves pass to the sensory cells of the posterior cor- 

 nua and decussate throughout the entire length of the cord (Brown-Sequard). 

 If the cord be divided longitudinally in the median line, there is complete 

 paralysis of sensation on both sides in all parts below the section (Fodera, 

 1822, and Brown-Sequard). In this section, the only fibres that are divided 

 are those passing from one side of the cord to the other. This decussation 

 is by fibres prolonged from the cells of the posterior cornua, which cross in 

 the gray commissure, around the central canal. 



When one lateral half of the cord is divided in a living animal, sensibil- 

 ity is impaired or lost on the opposite side of the body, below the section, 

 but there is hyperassthesia on the side corresponding to the section. The 

 exaggeration of sensibility has not been satisfactorily explained. 



Relations of the Posterior White Columns of the Cord to Muscular Co- 

 ordination. It was noticed by Todd, many years ago (1839-1847), in cases 

 of that peculiar form of muscular inco-ordination now known as locomotor 

 ataxia, that the posterior white columns of the cord were diseased. Eeason- 

 ing from this fact, Todd made the following statement with regard to the 

 office of these columns : 



" I have long been impressed with the opinion, that the office of the pos- 

 terior columns of the spinal cord is very different from any yet assigned to 

 them. They may be in part commissural between the several segments of 

 the cord, serving to unite them and harmonize them in their various actions, 

 and in part subservient to the function of the cerebellum in regulating and 

 co-ordinating the movements necessary for perfect locomotion." 



The view thus early advanced by Todd has been sustained by the results 

 of experiments on living animals. If the posterior columns be completely 

 divided, by two or three sections made at intervals of about three-fourths of 

 an inch to an inch and a quarter (20 to 30 mm.), the most prominent effect 

 is a remarkable trouble in locomotion, consisting in a want of proper co-ordi- 

 nation of movements (Vulpian). Experiments upon the different columns 

 of the cord in living animals, however, are so difficult that physiologists have 

 preferred to take the observations in cases of disease in the human subject as 

 the basis of their ideas with regard to the office of the posterior white col- 

 umns. 



The characteristic phenomenon of locomotor ataxia is inability to co-ordi- 

 nate muscular movements, particularly those of the extremities. There is 

 not of necessity any impairment of actual muscular power ; and although 

 pain and more or less disturbance of sensibility are usual, these conditions 

 are not absolutely invariable and they are always coincident with disease of 

 sensory conductors. The characteristic pathological condition is disease of 

 the posterior white columns (columns of Burdach). This is usually followed 

 by or is co-existent with disease of the posterior roots of the spinal nerves 



