36 



HE FLEX ACT I <>X. 



Masses of nerve-cells make up Nerve Centers, or Ganglia, 

 such as we saw on the dorsal roots of the spinal nerves. 

 These also would show under the microscope that their chief 

 constituent is a collection of nerve-cells which give off one or 

 more branches. 



The gray matter of the spinal cord is considered a collec- 

 tion of ganglia. We see that the outer layer of the brain is 

 grayish in color. Within is white matter, consisting of nerve 

 fibers that connect the cells of the gray layer with the various 



Branched Processes 



Unbranched Process, or 

 Axis Cylinder Process, 

 continued into Axis Cylinder 

 of a Motor Fiber 



Fig. 13. A Large Nerve-Cell from the Gray Matter of the Spinal Cord. 



parts of the body through the base of the brain, the spinal 

 cord, and spinal nerves. 



Reflex Action of the Spinal Cord of the Frog. Be- 

 fore we experiment on the frog, let us recall some facts that 

 we all know. After a fowl's head is cut off it " flops" around 

 for some time, and it may even jump clear from the ground. 

 If one takes hold of its feet to pick it up, it may begin to 

 struggle as if it were trying to get away. 



Now, we know that the bird cannot feel anything after its 

 head is cut off, not even if a pin were thrust into its body, 



