THE NERVOUS SYSTEM. 



'93 



THE SPINAL CORD. 



The spinal cord extends from the foramen magnum 

 through the vertebral canal. It is more or less cylindrical 

 throughout and has a diameter of about one centimeter 

 except in the lumbosacral region, where it grows gradu- 

 ally smaller until it is only one or two millimeters in 

 diameter. In order to study the cord, one should have a 



n 



FIG. 99. PHOTOMICROGRAPH OF CROSS SECTION OF HUMAN SPINAL CORD. 



a, Arachnoid membrane ; an, ventral side in which appears the deep 

 anterior fissure ; d, dura mater ; n, one of several nerve bundles ; 

 the tiny white spot in the center of the dark gray matter is the 

 canalis centralis; p, pia mater. X 5. 



mounted transverse microscopic section, and an entire cord 

 in situ with the dorsal wall of the vertebral canal removed, 

 so that the exit of the spinal nerves may be seen. 



The membranes (Fig. 99) which envelop the spinal cord 

 18 



