84 THE NERVOUS SYSTEM 



primarily responsible for these differences. One of these is the variation in the 

 size of the nerve roots at the different levels; for where great numbers of nerve- 

 fibers enter, they cause an increase in the size of the cord and particularly in 

 the volume of the gray matter. It has already been pointed out that the cer- 

 vical and lumbar enlargements are directly related to the large nerves supply- 

 ing the extremities. The second factor is this: Since all levels of the cord are 

 associated with the brain by bundles of long fibers, it is obvious that such long 

 fibers must increase in number and the white matter increase in volume as we 

 follow the cord from its caudal end toward the brain. All this is well illus- 

 trated in a diagram by Donaldson and Davis reproduced in Fig. 59. 



The outline of a section of the spinal cord at the fourth sacral segment is some- 

 what quadrilateral. The total area is small and the greater part is occupied 

 by the thick gray columns (Fig. 60). The size of the cord is much greater at 

 the level of the first sacral and fifth lumbar segments, as might be expected from 

 the large size of the associated nerves (Figs. 57, 60). There is both an absolute 

 and a relative increase in the white substance, which here contains the long 

 paths connecting the sacral portions of the spinal cord with the brain. Both 

 the anterior and posterior columns are massive, and the anterior presents a 

 prominent lateral angle. The large nerve-cells in the lateral part of the an- 

 terior column give rise to the fibers which run to the muscles of the leg. At the 

 level of the seventh thoracic segment (Figs. 56, 60) the cross-sectional area is less 

 than in the lumbar enlargement. Corresponding to the small size of the tho- 

 racic nerves the gray matter in this region is much reduced, both anterior and 

 posterior columns being very slender. The apex of the latter is some distance 

 from the surface and its cervix is thickened by a column of cells known as the 

 nucleus dorsalis. The columna lateralis is prominent. The white matter is 

 somewhat more abundant than in the lumbar region, and increases slightly in 

 amount as we follow the cord rostrally through the thoracic region (Fig. 59). 



A transverse section at the level of the seventh cervical segment is elliptic in 

 outline and has an area greater than that of any other level of the cord (Figs. 

 55, 60). The white matter is voluminous and contains the long fiber tracts 

 connecting the brain with the more caudal portions of the cord. The gray 

 matter is also abundant, as we might expect from the large size of the seventh 

 cervical nerve. The ventral column is especially thick and presents a prominent 

 lateral angle. The large laterally placed nerve-cells of the anterior column are 

 associated with the innervation of the musculature of the arm. The posterior 

 column is relatively slender, but reaches nearly to the dorsolateral sulcus. 



