30 PHYSIOLOGY. 



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

 lection of ganglions. We see that the outer layer of the 

 brain is grayish in color. Within is white matter, consist- 

 ing of nerve fibers that connect the cells of the gray layer 



Fig. 12. A Large Nerve Cell from the Gray Matter of the Spinal Cord. 



with the various parts of the body through the base of the 

 brain, the spinal cord, and spinal nerves. 



No Sensation without the Brain. 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 escape. 



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

 cut off, because the body is completely separated from the brain, which 

 is the center of sensation. So with the frog. After its head is cut 

 off, it cannot feel anything. 



Reflex Action of the Spinal Cord of the Frog. A frog may be 

 killed as directed on p. 9. Cut off its head and suspend the body 

 from any convenient support, such as the ring of a retort stand. 



1. On pinching the toes the foot will be drawn up. 



2. The sciatic nerve should now be severed as before directed 

 (p. 9). At the instant of cutting the nerve the muscles below will 

 twitch, because the nerve fibers running to them are stimulated. 



