362 THE SPINAL CORD. 



(of Gambault and Phillipe). Fibers from the spinal nerves enter 

 this postero-medial descending tract in the cervical, thoracic, 

 and, at least, the upper lumbar segments. After a variable course 

 within the tract, the fibers plunge forward into the posterior 

 columna of gray substance where they terminate. 



The descending postero-lateral tract (Fig. 103), situated 

 at the posterior surface of the cord medial to the entry zone, in 

 the lumbo-sacral region, appears to have been first described by 

 Thiele and Horsley. It is very largely endogenous, but contains 

 a number of root-fibers (Collier). In section it is triangular. 

 Its fibers terminate in the posterior columna of gray substance. 

 Both the descending radicular tracts are intermingled with endog- 

 enous fibers that belong to the fasciculus proprius. 



Posterior Fasciculus Proprius. (Fasciculus posterior pro- 

 prius}. This is made up of association fibers that connect different 

 segments of the cord. It is not a compact strand. Its fibers are 

 scattered throughout the posterior column, but they are especially 

 numerous in the descending radicular tracts, in the region along 

 the ventral one-third of the septum (the bandelette of Hoche) 

 and in the cornu commissural field of Marie (Figs. 102 and 103). 



The cornu commissural tract (Fig. 103) is placed between 

 the posterior columna (cornu), the posterior commissure and the 

 posterior septum. It extends up to the eleventh thoracic segment 

 and downward to the end of the cord. It contains both ascending 

 and descending fibers as do other parts of the fasciculus proprius. 



Lesions in the posterior columns cause disturbances of the 

 muscular and tactile senses, and ataxia and incoordination result. 

 If the entry zone is involved there is disturbance of all kinds of 

 common sensation, at the level of the lesion. These columns 

 are usually involved, by extension from the posterior roots, in 

 locomotor ataxia (posterior sclerosis), hence the paraesthesia, 

 crises, loss of reflexes, disturbed equilibrium and ataxic gait. 



ROOTS OF THE SPINAL NERVES. 



Thirty-one pairs of spinal nerves connect the cord with the per- 

 iphery. Each nerve is joined to the cord by two roots: an ante- 



