THE DISSECTION OF THE BACK 87 



straight line and at the bottom of a slight furrow. The fila 

 of the anterior root, on the other hand, are not so regularly 

 placed. They emerge from the medulla spinalis over an area 

 of some breadth. The portion of the medulla spinalis which 

 stands in connection with a pair of nerves receives the 

 name of a "neural segment." 



It will be noted that the size of the nerve-roots differs greatly. 

 The lower lumbar and upper sacral nerve-roots are much 

 the largest, whilst the lower sacral and the coccygeal roots 

 are the smallest. 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 



6' 



7' 



FIG. 24. A segment of the Medulla Spinalis ; anterior aspect. 

 (Schwalbe, after Allen Thomson. ) 



1. Anterior median fissure. 6. Posterior nerve-root. 



2. Posterior median sulcus. 6'. Spinal ganglion. 

 3 and 5. Fila of anterior nerve-root. 7. Anterior ramus. 



4. Posterior lateral groove. 7'. Posterior ramus. 



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 n-erve-roots also show great differences. 

 The differences are 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 and almost horizontally. Below the upper cervical 

 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 the bundle which 

 is called the cauda equina. 



