THE ROOTS OF THE PERIPHERAL NERVES, ETC. 169 



fibres (3, in Fig. 102) pass 'directly through the substantia gela- 

 tinosa Rolandi, which lies at the tip of the posterior horn, and 

 at once turn upward and downward, and only pass farther into 

 the gray substance at other levels. The fine fibres (4, in Fig. 

 10'2) become separated from the coarse immediately after the 

 entrance of the root, and pass as a separate bundle upward and 

 downward between the tip of the substantia gelatinosa and the 

 periphery. The cross-section of this fasciculus was called zona 

 terminalis by Lissauer, to whom we are mainly indebted for our 

 knowledge on this point. Fibrils are continually given off from 

 the zona terminalis to a net-work lying between it and the sub- 

 stantia gelatinosa, This net-work is called the zona spongiosa, 

 and from it fine fasciculi arise, which pass into the substantia 

 gelatinosa and are lost in the maze of fibres which make up the 

 main mass of the posterior horn. From this intricate maze of 

 fibres are developed new fasciculi, which pass across to the 

 opposite side through the anterior and posterior commissures, 

 and, emerging from the gray matter, are continued upward to the 

 brain in the ground-bundles of the anterior and lateral columns. 



AVe have, therefore, distinguished two paths in which 

 the fibres of the posterior roots are continued upward, a 

 direct one, in the posterior columns, and an indirect path by 

 which, after passing through the posterior horn, in some un- 

 known manner, the fibres cross to the opposite side. We 

 shall see later that the direct fibres, after passing through a 

 nucleus in the medulla oblongata, also decussate. 



It would not have been possible for me to recognize this 

 arrangement of the fibres of the posterior root were it not for 

 the fact that, in the lower vertebrates, the order of the fibres in 

 the spinal cord is a very simple one : I might almost^ call it 

 diagrammatic. Once it was shown, in the case of these animals, 

 that the majority of the posterior roots pass through a net-work 

 and afterward decussate before continuing on their course to 

 the brain, it was easy to recognize the same conditions in human 

 beings and other mammals. 



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