COMPRESSION OF THE SPINAL COED. 231 



Pupil dilator fibres, arising in the neighbourhood of the third 

 cranial nerve nucleus, are known to pass downwards in the 

 lateral columns of the cord to emerge by the first and second 

 dorsal roots. Thence they pass to the inferior cervical ganglion, 

 and so by the cervical sympathetic chain to the eye. Unilateral 

 pressure on the cervical cord may affect the fibres descending in 

 it and give rise to unilateral myosis. 



The cardio-accelerator fibres in the cord pursue a similar course 

 to the pupil dilator fibres but emerge lower in the fourth to the 

 ninth dorsal roots. Compression of the cord may irritate or 

 paralyse these fibres, and acceleration or slowing of the heart 

 rate will result. 



Vaso-dilatation may occur in acute compression. The vaso- 

 dilator fibres for the whole body leave the spinal cord in the 

 middle of its extent, probably reaching from about the second 

 dorsal to the second lumbar nerves. 



The vaso-constrictor nerves have a much more extensive 

 outflow. 



Since secretory fibres for the sweat glands also leave the cord 

 these are liable to be paralysed where nerve roots are compressed, 

 and so the injection of pilocarpine has been used to determine 

 the level of compression, the sweat ceasing at this line. 



The upper level at which sensation is in any way altered 

 is usually taken to indicate the level of the compressing 

 lesion. The level thus determined often proves too low. 

 Owing to the oblique course of the nerve roots in the spinal 

 canal before they emerge, the seat of compression is in 

 most cases higher than the upper limit of anaesthesia, the 

 difference corresponding to the vertical length of the nerves in 

 the canal. Even when this is allowed for by the use of suitable 

 tables, the level of the anaesthesia may prove to be considerably 

 below the level of the lesion, and this is due to the fact that 

 sensory root symptoms may be absent at the level of the lesion 

 and that all the sensory fibres ascending in the cord from the 

 parts below have not suffered equally, but that the longest 

 tracts have first ceased to conduct. 



