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A MANUAL OF PHYSIOLOGY 



cord as the massive crossed pyramidal tract of the opposite 

 side. A few (usually about 10 per cent.) remain on the same 

 side as the slender direct pyramidal tract. The size of this tract 

 varies much in different individuals, and it is occasionally absent. 

 Its breadth 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, con- 

 tinually passing off into 

 the grey matter to make 

 connection (p. 775) with 

 the cells of the anterior 

 horn, whose axis-cylinder 

 processes enter the an- 

 terior roots of the spinal 

 nerves. The losses which 

 it suffers as it descends 

 the cord may be in some 

 slight degree compensated 

 by the bifurcation of some 

 of its fibres (geminal 



FIG. 327. MOTOR PYRAMIDAL TRACTS (DIA- 

 GRAMMATIC) (HALLIBURTON, AFTER GOWERS). 



The convolutions are supposed to be cut 



in vertical transverse section, the internal fibres), but ultimately the 

 capsule, I, C, and the crus in horizontal 

 section. O, TH, optic thalamus ; CN, cau- 

 date nucleus ; La and LS, middle and ex- 

 ternal portions of lenticular nucleus ; /, a, I, 

 fibres from the face, arm, and leg areas of the 

 cortex respectively ; E, S, Sylvian fissure. The 

 genu or knee of the internal capsule is indi- 



cated by the asterisk. 



whole tract forms synapses 

 with cells in the grey 

 matter, and dwindles away 

 as the lumbar region is 

 reached (Fig. 314). 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 corre- 

 sponding 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 represented by the fibres that degenerate in that tract after 

 a lesion in the ' motor ' area of the same side (p. 774). 



