870 THE CENTRAL NERVOUS SYSTEM 



fibres passing across the middle line from the gracile and cuneate nuclei 

 (sensory decussation or decussation of the fillet). The mosaic of grey 

 and white matter formed in the medulla by the interlacing of longi- 

 tudinal and transverse fibres with each other and with the relics of the 

 anterior horn, is called the reticular formation (formatio reticularis). 

 It occupies the anterior and lateral portions of the bulb behind the 

 pyramids and olivary bodies, and is continued upwards in the dorsal 

 portion of the pons and crura cerebri, and downwards for a little way 

 into the upper part of the cervical cord. 



The cerebro-spinal axis passes up from the medulla through the pons, 

 encircled and traversed by the transverse pontine fibres derived from 

 the middle cerebellar peduncle or commissure, which enclose every- 

 where 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 divergent 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 dis- 

 persed, 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 pyramidal tract, and a dorsal portion, or tegmentum, 

 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. 352). A portion of the tegmentum is 

 continued below the optic thalamus. 



SECTION V. CONNECTIONS OF THE LONG PATHS OF THE CORD. 



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. 797), degenerate above the section. Since the cell- 

 bodies of these neurons lie in the ganglion, 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 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 degene- 

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



