190 



HEAD AND NECK 



the larger, whilst the lower sacral and the coccygeal roots 

 are the smaller. In the cervical region the roots increase in 

 size from above downwards, but more rapidly in the lower 

 members of the group ; in the thoracic region the roots of the 

 first nerve are large, but those which succeed it are small and 

 of uniform size. 



In relative length, and in the direction which they follow in 

 the vertebral canal, the nerve-roots also show great differences. 

 This is due to the medulla spinalis being so much shorter than 

 the canal in which it lies. In the upper part of the cervical 

 region the nerve-roots are short, and proceed laterally in a 

 more or less horizontal direction. Below the upper cervical 



FIG. 77. A segment of the medulla spinalis ; anterior aspect. 

 (Schwalbe, after Allen Thomson. ) 



1. Anterior median fissure. 



2. Posterior median sulcus. 



3 and 5. Fila of anterior nerve-root. 

 4. Posterior lateral groove. 



6. Posterior nerve-root. 

 6'. Spinal ganglion. 



7. Anterior branch. 

 7'. Posterior branch. 



region the nerve-roots become more oblique, and the lower 

 the origin of the nerve the longer is its course in the canal. 

 The arrangement of the lower thoracic, the lumbar, sacral, and 

 coccygeal nerve-roots is particularly characteristic. They are 

 exceedingly long, and descend vertically from the lower 

 portion of the medulla spinalis, forming a bundle which is 

 called the cauda equina. 



Mode of Exit of Spinal Nerves from Vertebral Canal. 

 The lower six cervical nerves, the thoracic nerves, and the 

 lumbar nerves make their exit through the intervertebral 

 foramina ; whilst each of the two branches of the upper four 

 sacral nerves finds its way out by a sacral foramen. The 

 upper two cervical nerves, the fifth sacral nerve, and the 

 coccygeal nerve, however, follow a different course. The 

 sub-occipital emerges by passing over the posterior arch of 



