ZOOLOGY. 



360. Peripheral Nervous System Spinal Nerves. 

 Groups of nerve fibres spring from the gray matter of the 

 cord and pass to the organs of the body. These nerves arise 

 in pairs one pair to each body segment and pass out be- 



FIG. 171. Diagram of a cross-section of the spinal cord through the roots of spinal 

 nerves. Drawn by Folsom. c, central canal; d.f., dorsal fissure; d.r., dorsal root of 

 spinal nerve arising from the dorsal horn of the gray matter (g) ; gn., ganglion on the 

 dorsal root; n, spinal nerve; v.f., ventral fissure; i\r., ventral root of the spinal nerve, 

 arising from the ventral horn of the gray matter; w., white matter. (The dorsal fissure 

 in the diagram is broader than it should be.) 



Questions on the figure. What is the structural difference between the 

 white and gray matter in the cord? Describe their arrangement. How 

 are the two halves of the cord united? Which are sensory and which 

 motor roots? What structural differences do you notice in the roots? 



tween the vertebrae. Each nerve has two " roots," a dorsal 

 and a ventral, from each of which some of its fibres come 

 (Fig. 171, d.r., v.r.). The roots differ in appearance in that 

 the dorsal has an enlargement (ganglion) containing nerve 

 cells ; the ventral has none. The fibres from these two roots 

 combine to form the nerve, but each fibre remains independent 

 throughout. It is known by experiment that the fibres of the 

 dorsal root carry impulses toward the spinal cord (" sensory ") 

 and those of the ventral root carry impulses from the cord 

 (" motor"). In certain regions the nerves springing from 

 successive segments of the body may have numerous inter- 

 lacing fibres, forming what is known as a plexus. 



361 . Cranial Nerves. Those nerves arising from the brain, 

 that is, inside the cranium, are called cranial nerves. There 

 are ten to twelve pairs of these, but they are not of equal mor- 



