ARRANGEMENT OF NERVE CELLS IN THE SPINAL CORD 543 



The nerve tracts of the cord are made up of medullated nerve fibers of 

 different sizes, arranged longitudinally, and of a supporting material of 

 ordinary fibrous connective tissue and neuroglia, figure 105. 



The general rule respecting the size of different segments of the cord 

 appears to be that each is in direct proportion in this respect to the size and 

 number of nerve roots given off from it, and has but little relation to the size 

 or number of those given off below it. Thus the cord is very large in the 

 middle and lower part of its cervical portion, whence arise the large nerve 

 roots for the formation of the brachial plexuses and the nerve supply of the 

 upper extremities; and again enlarges at the lowest part of its dorsal portion 

 and the upper part of its lumbar, at the origins of the large nerves which, 

 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 increased in the quantity of the gray matter; for there seems reason 

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

 large 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 

 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 meshwork of nerve fibers, most of which are very fine and delicate. 

 Medullated fibers mingled with the small gray fibers about the borders of the 

 gray substance. Mingled with it and supporting it is the meshwork of the 

 neuroglia. 



The multipolar cells of the cord are either scattered singly or arranged in 

 groups or columns in bilateral symmetry. The following are to be distin- 

 guished, certain of the groups being more or less marked in all of the regions 

 of the cord, viz., a, those in the anterior columns; b, those in the lateral 

 columns; c, those in the posterior columns; and d, intrinsic cells distributed 

 throughout the gray matter. 



The cells in the anterior columns are large and branching, and each gives 

 rise to an axis-cylinder process which passes out in the anterior nerve root. 

 These cells are everywhere conspicuous, but are particularly numerous 

 in the cervical and lumbar enlargements. In these districts they may be 

 divided into several groups; i. A group of large cells close to the tip of the 

 inner part of the anterior column, i, figure 362. This group is called the 

 antero-mesial group of motor cells. It forms a column the full length of the 



