768 A MANUAL OF PHYSIOLOGY 



strands of fibres as they sweep through it from the lateral pyra- 

 midal tract to take up a position in the pyramid of the opposite side 

 (decussation of the pyramids), and a little higher up by 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 longitudinal 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 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 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 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, con- 

 taining 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. 324). A 

 portion of the tegmentum is continued 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. 693), 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 degenera- 

 tion 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 de- 

 generation. 



When a number of consecutive posterior roots are cut, the 



