THE NERVOUS SYSTEM 115 



former is the source of the nerves entering the brachial 

 plexus, the latter of the fibers going to the sciatic plexus. 

 Remove the cord from the neural canal, strip off the 

 meninges, and note that the cord rapidly diminishes in 

 size in the posterior lumbar region, finally terminating in 

 the slender thread-like filum termmale. The posterior 

 spinal nerve roots proceed caudally, accompanied by the 

 filum terminale, for a considerable distance from their at- 

 tachment to the cord. These roots constitute the ccmda 

 equina, so called from its resemblance to a horse's tail. 



Exercise XXXI. Draw a cross section of the spinal cord to 

 show the origins of the spinal nerves, the spinal ganglia, etc. 



