68o A MANUAL OF PHYSIOLOGY 



everywhere between them numerous collections of nerve-cells (nuclei 

 pontis). Enlarged by the accession of many of these fibres which 

 come from the cortex of the cerebellum on the opposite side, as well 

 as of fibres from the nuclei of the cranial nerves that take origin in 

 this neighbourhood (fifth and eighth), the central nervous stem 

 bifurcates above the pons into the two diverging crura cerebri. 

 From each crus a great sheet of fibres passes up between the optic 

 thalamus and the caudate nucleus of the corpus striatum on the one 

 hand, and the globus pallidus of the lenticular nucleus on the other, 

 as the internal capsule, from which they are dispersed, in the corona 

 radiata, to the cerebral cortex. Both in the upper part of the pons and 

 in the crus a ventral portion, or crusta, containing the fibres of the pyra- 

 midal tract, and a dorsal portion, or tegmcntum, can be distinguished, 

 the line of separation being marked in the crus by a collection of 

 grey matter, called from its usual, though not invariable, colour the 

 substantia nigra (Fig. 249). A portion of the tegmentum is con- 

 tinued below the optic thalamus. 



Coming back now to our question as to the connections 

 of the long tracts of the cord, let us consider, first of all, 



The Connections of the Postero-median and Postero-external 

 Columns. When a single posterior root is divided, say in the 

 dorsal region, between the cord and the ganglion, its fibres, 

 as we have already seen (p. 606), degenerate above the section. 

 If a series of microscopic sections of the spinal cord be made, 

 well-marked degeneration will be found at the level of 

 entrance of the root on the same side of the cord, while 

 below that level there will be only a few degenerated fibres 

 in the comma tract. Immediately above the plane of the 

 divided root the degeneration will be confined to Burdach's 

 column and to its external border. Higher up it will be 

 found in the internal portion of Burdach's and the external 

 rim of Coil's column. Still higher up the degenerated fibres 

 will be confined to the postero-median column ; the postero- 

 external will be entirely free from degeneration. 



When a number of consecutive posterior roots are cut, the whole 

 of the postero-external column in the sections immediately above the 

 highest of the divided roots will be found occupied by degenerated 

 fibres, while Coil's column may be free from degeneration, or de- 

 generated only at its outer border. Higher up degeneration will be 

 found to have involved the whole of the postero-median column, and 

 to have cleared away altogether from the postero-external. The 

 degeneration in the column of Coll may be traced along the whole 

 length of the cord to the medulla, although the number of degene- 

 rated fibres diminishes as we pass upward. The explanation of 



