IRRITABILITY 113 



the face and the various parts of the sense organs 

 (Fig. 71.) 



The Work of Nerves. — The nerves form the only 

 protoplasmic connection between the skin and various 



Fig. 69.— Diagram of the nervous system Fig. 70.— Diagram of the 



of a starfish nervous system of an 



earthworm. 



muscles. When a nerve ending in the skin is stimulated 

 by some form of physical or chemical contact, the stimu- 

 lus passes down the nerve and gives rise to a sensation, or 

 to the contraction of a muscle, or to both. A sensory 

 nerve carries a stimulus to the ventral or front side of 

 the spinal cord. From there it may pass through the 

 spinal cord to the brain and give rise to a sensation ; and 

 it may also pass to the dorsal side of the spinal cord 

 and back to the muscles through a motor nerve and give 

 rise to a contraction. Sometimes the passage is so quick 

 from the point of contact to the muscle that the stimulus 

 does not have time to go to the brain first. The move- 

 ment then takes place before the mind knows anything 



