398 



PHYSIOLOGY 



pyramidal decussatioii. The anterior pyramids represent a certain 

 number of fibres which have not crossed with the others, but continue 

 the course of the medullary pyramids for a time, crossing gradually by 

 the anterior commissure on their way down the cord, so that, as a 

 rule, they come to an end in the mid- dorsal region, all the fibres 

 having passed into the lateral columns of the opposite side. A few 

 fibres of the pyramids on their way from the cerebral cortex pass into 

 the lateral columns of the same side ; these are the uncrossed pyra- 

 midal fibres. The greater number of the fibres, however, finally reach the 

 crossed pyramidal tracts, in which they can be traced as far as the lower 

 end of the cord. They end in the spinal cord by turning into the grey 

 matter and there breaking up into a fine bunch of fibrils in close 



FIG. 175. Diagram (from SCHAFER) showing the ascending (right side) 



and the descending (left side) tracts in the spinal cord. 

 1, crossed pyramidal; 2, direct pyramidal; 3, antero -lateral descending ; 

 3a, spino-olivary descending (bundle of Helweg) ; 4, pre-pyramidal (rubro- 

 spinal) ; 5, comma ; 6, postero-mesial ; 7, postero-lateral ; 8, Lissauer's 

 tract ; 9, dorsal (ascending) cerebellar ; 10, antero -lateral ascending ; 

 sm, septo- marginal ; spl, dorsal root zone ; a, anterior horn-cells ; i, inter - 

 medio-lateral horn ; p, cells of posterior horn ; d, Clarke's column. The 

 fine dots represent the situation of the ' internuncial ' or ' endogenous ' 

 fibres of the spinal cord. 



connection with the motor-cells of the anterior cornu, or, according 

 to Schafer, with the cells of the posterior horn. 



On their way down the cord they give off fine side branches or 

 collaterals, which run into the grey matter, thus establishing connec- 

 tions between one cortical cell and the anterior cornual cells of several 

 different segments of the spinal cord. These fibres carry voluntary 

 motor impulses from the cerebral cortex to the reflex motor mechanisms 

 of the cord. Their destruction by disease, or otherwise, causes the 

 abolition of voluntary control over the muscles, without, however, 

 interfering with the reflex motor functions of the cord, which, as a 

 matter of fact, are increased in cases where these tracts have under- 

 gone degeneration. 



