582 



THE NERVOUS SYSTEM 



after forming the lumbar and sacral plexuses, are distributed to the lower 

 extremities. The chief cause of the greater size at these parts of the spinal 

 cord is increase in the quantity of the gray matter; for there seems reason 

 to believe that the white part of the cord becomes gradually and progressively 

 larger from below upward, doubtless from the addition of a certain number 

 of ascending fibers from each pair of nerves. 



From careful estimates of the number of nerve fibers in a transverse sec- 

 tion of the cord toward its upper end, and the number entering or issuing 

 from it by the anterior and posterior roots of each pair of nerves, it has been 

 shown that in the human spinal cord not more than half of the total number 



FIG. 361. From the Lower Lumbar Cord of Man, after a Preparation by Klonne and 

 Muller stained by Weigert and Pal's method. A portion of the gray substance of the 

 anterior column with the adjoining portions of the lateral funiculus is represented, showing 

 anterior column cells and the fine medullated fibers which enter the gray substance from the 

 lateral funiculi and surround the nerve cells, which here are provided with fine pigmented 

 granules. High power. (Kolliker.) 



of nerve fibers of all the spinal nerves are contained in a transverse section 

 near its upper end. It is obvious, therefore, that at least half of the nerve 

 fibers entering it must terminate somewhere within the cord itself. 



The Arrangement of Nerve Cells in the Spinal Cord. The gray mat- 

 ter of the spinal cord consists of numerous groups of nerve cells and of a 

 close mesh work of nerve fibers, most of which are very fine and delicate. 



