IRRITABILITY 



111 



so associated that if one contracts the other relaxes. On 

 each side of a hinge joint is a mnscle attached to both 

 bones; one bends the joint when it contracts, the other 

 straightens the joint when it contracts. In moving the 

 eye to the right, the outside muscle of the right eye con- 

 tracts while the inside muscle relaxes (Fig. 67). At 



Fig. 67.— The eye and its muscles, a, pulley of upper rotating muscle ; b, part of 

 skull (eye socket); c, muscle of upper lid; d, upper rotating muscle ; e, muscle 

 turning eye upward; /, muscle turning eye outward; g, muscle turning eye 

 downward; h, bone of socket ; i, optic nerve; k, lower rotating muscle. (From 

 Fitz.) 



the same time, the inside muscle of the left eye contracts 

 while the outside muscle relaxes. Both eyes thus move 

 together. 



Relation of Nerves to Muscles. — If the finger touches 

 a hot stove, the muscle that jerks it away is likely 

 to be one of those in the upper arm. This muscle 

 did not come in contact with the stove, yet the stimulus 

 reached it. There must therefore be a passageway by 

 which a stimulus applied to the skin can reach a muscle 

 far removed from the point of contact. This connecting 

 passageway is furnished by the nerves. 



