CROSSED PARALYSIS 39 



As the pyramidal tract passes down through the brain-stem, 

 the fibres which are destined for the nuclei of the motor 

 cerebral nerves leave it at different points and cross the 

 median plane to reach their objective (Fig. 22). In doing 

 so they decussate with the corresponding fibres of the opposite 

 side. This arrangement, together with the fact that the 

 pyramidal tract is placed at no great distance from the cerebral 

 motor nuclei, offers an explanation for the occurrence of 

 crossed paralysis. 



The nucleus of the third cerebral (oculo-motor] nerve is 

 placed in the mid-brain opposite the superior corpora quadri- 

 gemina, and the fibres which it obtains from the pyramidal 

 tract decussate at a slightly higher level. If a small localised 

 haemorrhage occurs in the mid-brain at the level of the oculo- 

 motor nucleus, it will involve the pyramidal tract after it has 

 given off its fibres to the third nucleus of the opposite side. 

 Such a haemorrhage, however, may readily involve the oculo- 

 motor nucleus on the same side, and a crossed paralysis results. 

 The lower cerebral nerves and the spinal nerves are paralysed 

 on the opposite side of the body, while the oculo-motor nerve 

 is paralysed on the same side as the lesion. The oculo-motor 

 nerve of the opposite side escapes because the lesion occurs 

 below the point where its fibres leave the pyramidal tract and 

 is not large enough to affect the structures in the opposite half 

 of the mid-brain. This variety of crossed paralysis is some- 

 times referred to as the syndrome of Weber. 



Theoretically, similar crossed paralyses may occur affecting 

 any one of the motor cerebral nerves, but the only one of 

 common occurrence is that in which facial paralysis exists on 

 one side while the limbs are paralysed on the opposite side of 

 the body. In this case, the lesion is placed in the pons at the 

 level of the facial nucleus, and it is complicated by the fact 

 that the nucleus of the sixth nerve and the sensory nucleus 

 of the fifth are also likely to be involved (Fig. 23). The 

 condition is described in detail on page 86. 



The structure of the various parts of the brain-stem and the 



