434 



THE INTERNAL STRUCTURE 



stance of the 'pons Varolii ' (fig. 154), the sites in which 

 decussation of motor channels takes place, are altogether 



limited in area when com- 

 pared with what obtains for 

 sensor}^ channels. 



The longitudinal fibres of 

 the Spinal Cord are in the 

 main divisible (if we exclude 

 those specially in relation 

 with the Cerebellum) into 

 three categories, viz., (1) 

 fibres transmitting ' ingoing ' 

 currents towards the Brain ; 



(2) fibres which transmit 

 ' outgoing ' currents ; and 



(3) fibres of a ' commissural ' 

 order, serving to connect 

 separate groups of cells or 

 centres in different parts of 

 the Spinal Cord itself, or in 

 the Spinal Cord and Me- 

 dulla. 



The Spinal Cord being, 

 moreover, a bilaterally sym- 

 metrical organ, the groups 

 of cells above referred to 

 are similarly represented in 

 each half of it (fig. 19) ; and 

 the similar Motor and Sen- 

 sory regions of these two halves of the Cord and Medulla 

 are to a considerable extent brought into structural relation 

 with one another by means of numerous transverse ' com- 

 missural ' fibres. ' 



The first two sets of longitudinal fibres, above referred 



Fig. 154. — Diagram illustrating the 

 place and mode of ' decussation ' of 

 Motor Fibres in the Medulla and in the 

 Pons. (i3roadbent.) B, B', two sets of 

 nuclei of brachial plexus, not con- 

 nected by transverse commissures ; 

 O, O', two sets of oculo- motor nuclei in 

 Pons, freely connected with one another 

 by transverse commissural fibres. 

 S, S', motor fibres from Corpus Striatum. 



