THE MASTER TISSUES 



49 



ticle to particle. It resembles electricity somewhat, but 

 some physiologists think it is like a wave of chemical 

 change, running along the nerve faster than a railway 

 train can run. It reaches a nerve cell in the spinal cord. 

 The disturbance^ there causes the cell to send out impulses 

 along its other branches or 

 fibers. Some impulses are 

 sent down the arm again to 

 its muscles, causing them 

 to contract, and the arm is 

 jerked away, as we say, by 

 reflex action, or action with- 

 out will on our part. Other 

 impulses go at the same time 

 to the brain, and we become 

 conscious of what has hap- 

 pened. The nerves which 

 carry impulses to the nerve 

 cells are called sensory 

 nerves, or nerves of feeling, and those which carry 

 impulses from the cells to the muscles are called motor 

 nerves, or nerves of motion. Nerves transmit impulses, 

 but do not originate them. An impulse in a nerve can be 

 excited by pinching, pricking, electricity, a drop of acid, 



FIG. 47. A Nerve Cell. 



From the gray matter of the spinal cord, 

 magnified. 



FIG. 48. Nerve Fiber, highly magnified. 

 Showing axis (gray), sheath of fatty material (black), sheath of connective tissue (white). 



a hot wire, a cold object, or a thought. Reflex action 

 always occurs on account of some influence from the outer 

 world, but voluntary action comes from activity in the brain. 

 80. Suppose you step out of a warm house into a cold 

 wind. The face immediately blanches or turns white. 

 Let us see how this is accounted for. There are muscle 

 fibers in the walls of the blood vessels. The cold air 



