THE SPINAL COED AND THE SPINAL NERVES. 87 



tween the spinal cord and the brain, the one passing 

 gradually into the other. In its course the cord presents 

 two expansions, an upper, 10 (Fig. 80), the cervical en- 

 largement, reaching from the third cervical to the first 

 dorsal vertebrae, and a lower or lumbar enlargement, 9, 

 opposite the last dorsal vertebrae. 



Kunning along the middle line on both the ventral and 

 the dorsal aspects of the cord are fissures which (C, Fig. 

 81) nearly divide it into right and left halves. 



A transverse section, 0, shows that the substance of the 

 cord is not alike throughout, but that its white superficial 

 layers envelop a central gray substance arranged somewhat 

 in the form of a capital H. Each half of the gray matter 

 is crescent-shaped, and the crescents are turned back to 

 back and united across the middle line by the gray com- 

 missure. 



The Spinal Nerves. Thirty-one pairs of spinal nerves 

 join the spinal cord in the neural canal of the vertebral 

 column, entering the canal through the intervertebral fora- 

 mina (p. 32). Each divides in the foramen into a dorsal 

 and ventral portion, known respectively as the posterior 

 and anterior roots of the nerve (6 and 5, Fig. 81), and 

 these are attached to the sides of the cord. On each pos- 

 terior root is a spinal ganglion (6', Fig. 81), placed where 



Whafexpansions are seen in it? Where is each placed? 



How are the right and left halves of the cord separated? 



Is the spinal cord alike all through? What are the colors of its 

 outer and of its inner portions? What is the form of the gray mat- 

 ter of the cord as seen on cross-sections? How are the crescents 

 united? 



How many spinal nerves are there? What do they join? How 

 do they get into the neural canal? Where do they divide? What 

 are the divisions named? To what are they attached? What is 

 found on each posterior root? 



