688 A MANUAL OF PHYSIOLOGY 



10 per cent.) remain on the same side as the slender direct 

 pyramidal tract ; but the breadth of this tract constantly 

 diminishes as it proceeds down the cord, and it disappears 

 before the middle of the thoracic region is reached, its fibres 

 continually decussating across the anterior white commissure 

 and plunging into the opposite anterior horn. They either 

 end among its cells, or passing through it, reinforce the 

 crossed pyramidal tract. The fibres of this crossed tract 

 are, in their turn, continually passing off into the grey 

 matter, where they break up into terminal brushes, and thus 

 make connection either directly or through the interposition 

 of other neurons with the cells of the anterior horn, whose 

 axis-cylinder processes enter the anterior roots of the spinal 

 nerves. The losses which it suffers as it passes along the 

 cord may be in some slight degree compensated by the 

 bifurcation of some of its fibres (geminal fibres), but ulti- 

 mately the whole tract forms synapses with cells in the grey 

 matter, and dwindles away as the lumbar region is reached 

 (Fig. 239). It has been asserted that on their way 

 down the cord the two crossed pyramidal tracts exchange 

 some fibres with each other (recrossed fibres) ; and it was 

 supposed that this would explain the escape in hemiplegia 

 (paralysis of one side of the body) of those muscles which 

 are accustomed to work with the corresponding muscles on 

 the opposite side, e.g., the respiratory muscles. But although 

 there is no doubt that such muscles are innervated to some 

 extent from both cerebral hemispheres, this is due not to- 

 recrossed, but to uncrossed (homolateral) , fibres, which in the 

 cord run down in the lateral pyramidal tract, and are repre- 

 sented by the fibres that degenerate in that tract after a 

 lesion in the Rolandic area of the same side (p. 686). 



(2) A great afferent or sensory path by which some at least 

 of the impulses carried up through the posterior roots of the 

 spinal nerves, after passing through various relays of nerve- 

 cells, 're,ach the cortex of the cerebellum ; or the upper 

 portions of the central grey tube, the corpora quadrigemina 

 and optic thalamus ; or, finally (both indirectly and by a 

 more direct route which certain fibres of the mesial fillet 

 follow through the tegmentum and the posterior limb of 



