4 o 



THE NERVOUS SYSTEM 



results of lesions of the brain-stem will be dealt with when the 

 nuclei of the individual cerebral nerves are described. 



The Spinal Medulla (Spinal Cord) begins at the foramen 

 magnum, where it is continuous with the medulla oblongata, 

 and extends downwards through the vertebral canal to the 

 lower border of the first lumbar vertebra. In the lower 

 cervical and the lower thoracic regions, the spinal medulla 



Facial 



Motor fibres 

 FIG. 23. Transverse Section through the Pons (Diagrammatic). 



increases in size, and these localised areas of enlargement 

 correspond to the segments of origin of the great limb 

 plexuses. 



Owing to the relative disproportion in length between the 

 spinal medulla and the vertebral canal, the nerve-roots must 

 arise from the spinal medulla at a much higher level than the 

 intervertebral foramina through which they pass. Below the 

 lower end of the spinal medulla, therefore, the vertebral canal 

 is occupied by the nerve-roots of the lumbar, sacral, and 

 coccygeal nerves, which together constitute the cauda equina. 



The lower lumbar region of the vertebral canal is selected as 



