THE NERVOUS SYSTEM. 193 
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 
Fic. 99. PHotomicrocrary or Cross Section or HuMAN SPINAL Corp. 
a, Arachnoid membrane; an, ventral side in which appears the deep 
anterior fissure; d, dura mater; 1, one of several nerve bundles; 
the tiny white spot in the center of the dark gray matter is the 
canalis centralis; , pia mater. 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 
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