110 NERVOUS SYSTEM. 



thus pass on as white fibres in the lateral columns. These fibres 

 enter the lateral columns (of the opposite side), and pass to the 

 medulla as a distinct tract the antero-lateral ascending tract at 

 the periphery of the lateral, extending into the anterior column. 



What sensations are transmitted by this set of fibres ? 



It is by this tract that sensations of pain and of temperature are 

 supposed to pass. There are also afferent fibres in the posterior 

 columns the posterior median by which the sensations of touch 

 and weight (or muscular sensation) are believed to pass ; the latter, 

 however, does not decussate. To recapitulate : sensations of pain 

 and temperature are transmitted through the lateral columns, and 

 those of touch and weight in the posterior columns. 



What is the path of the motor impulses ? 



As has already been seen, these fibres are better demonstrated 

 than the sensory. Most of the motor fibres cross to the opposite 

 side in the medulla oblongata (decussation of the pyramids), and 

 the impulses pass down by the lateral columns in the crossed or 

 lateral pyramidal tract on the side opposite to that in which they 

 originate. There is also a set of motor fibres which do not cross, 

 but pass directly to the same side in the anterior columns, and 

 decussate in the anterior or white commissure near the point of 

 distribution. The destination of these fibres is variable, for the 

 reason that the amount of decussation in the medulla is not con- 

 stant ; but, as a rule, the fibres in the direct tract go to the upper 

 portion of the body. 



Do the motor tracts carry only the fibres arising from the brain ? 



No. The cells in the anterior cornu of the gray matter of the 

 cord originate many of the fibres which go to the nerves. This is 

 demonstrated after division of the cord by stimulating these fibres : 

 a series of co-ordinated motions follows, and this stimulus may be 

 applied direct to the fibres or through the sensory nerves. 



If one lateral half of the cord be cut through transversely, what 

 are the results? 



1. Motor. There will be paralysis of motion of one-half the body 

 below the section ; and the paralysis will be of the same side if the 

 section be below the decussation in the medulla ; if it be above, the 

 paralysis will cross to the opposite side. 



2. Sensory. Anaesthesia of the opposite side below the section, 



