IO2 



NERVOUS SYSTEM 



this be so the path consists in part of relays of shorter neurons (D6jerine, 



1914). 



The uncrossed path in the posterior funiculus for tactile impulses entering 

 the cord through any given dorsal root overlaps by many segments the crossed 

 path in the ventral funiculus (Fig. 230). Some of the uncrossed fibers even 

 reach the nuclei of the funiculus gracilis and funiculus cuneatus in the medulla 

 oblongata. This extensive overlapping of the uncrossed by the crossed paths 

 accounts for the fact that lateral hemisection of the human spinal cord rarely 

 causes marked disturbance of tactile sensibility below the lesion (Petren, 1902; 



Head and Thompson, 1906). 



I 



Ascending branch of dorsal root fiber - 

 Myelinated fiber of dorsal root^ 

 Spinal ganglion 



Unmyelinated fiber of dorsal root' 



~ Lateral spinothalamic tract 

 (pain and temperature) 



Ventral spinothalamic tract 

 (touch) 



Fig. 73. Exteroceptive pathways in the spinal cord. 



Since it seems clear that the dorsal root fibers subserving tactile sensibility ascend for 

 some distance in the posterior funiculus, they must be included among the myelinated fibers 

 of the medial division of the dorsal root, because only myelinated fibers ascend in that 

 funiculus. This conclusion is in keeping with the facts already mentioned concerning the 

 termination of myelinated fibers in the supposedly tactile end organs, such as Meissner's 

 corpuscles and Pacinian corpuscles. It is also in. keeping with facts to be mentioned in 

 a following paragraph concerning the structure of the median nerve. 



The Lateral Spinothalamic Tract. It seems to be well established that the 

 dorsal root fibers, which serve as pain conductors, terminate in the gray matter 

 almost at once after entering the cord, and come into synaptic relations with 

 neurons of the second order, whose axons run in the lateral spinothalamic tract. 

 From cells in the posterior column fibers arise, which in man cross to the opposite 

 side of the cord in the anterior white commissure and ascend in the lateral spino- 

 thalamic tract to end in the thalamus (Figs. 73, 231). This is a tract of ascending 



