ARRANGEMENT OF NERVE CELLS IN THE SPINAL CORD 



511 



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 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 section 

 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 



15 iS 



FIG. 358. Horizontal Section of the Cord and its Envelopes, at the Middle of a Vertebral Body 

 (Schematic), i, Spinal cord with 2, its anterior median fissure; 3, its posterior median fissure; 

 4, anterior roots; 5, posterior roots; 6, pia mater (in red); 7, ligamentum dentatum; 8, connect- 

 ing fibers passing from the pia to dura mater; 9, visceral layer, and 9', parietal layer of the arach- 

 noid (in blue); 10, subarachnoid space; n, arachnoid cavity; 12, dura mater (in yellow); 13, 

 periosteum; 13,' external periosteum; 14, cellular tissue situated between the dura mater and the 

 wall of the vertebral canal; 15, common posterior vertebral ligament; 16, intraspinal veins; 17, 

 vertebra in section. (Testut.) 



thjat 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 in 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 



