70 THE HUMAN MECHANISM 



of the opposing side from making a certain play. But there 

 is always a doubt whether he will do this thing or " lose 

 his head" and do something else, leaving his man free to do 

 what he pleases. In the latter case that organism which we 

 call a football eleven would act very much as the human 

 organism would act if it were to wink and not cough when a 

 foreign body lodges on the lining membrane of the larynx. 



Evidently we have something here to explain. Why are 

 the actions of the body purposeful; that is, adapted to ac- 

 complish the right thing at the proper time? And in the 

 more complicated actions how is the work of the different 

 units the organs and the cells adjusted, or coordinated', 



that is to say, how is each 

 one made to do its proper 

 share of the work ? Let 

 us begin with the study 

 of a very simple action 

 that of winking. 



1. Winking is caused by 



the contraction of muscle 

 FIG. 38. The muscular mechanism fiberg which mn trang _ 

 of winking 



versely across the eyelid 



in a curved course. As they are attached most firmly at 

 the regions A and B (Fig. 38), their shortening straightens 

 their arched course and so brings the two edges of the eye- 

 lid into contact. The work of this muscle is obviously pur- 

 poseful, for the wink takes place when the eyeball needs 

 protection; it is also coordinated, since the act is executed 

 by a number of fibers working together, for if only those of 

 the lower eyelid were to contract the lids could not be closed. 

 The muscle fibers which work together to produce the 

 wink do not originate their own activity. They merely do 

 what they are stimulated to do by the nervous impulse, 

 which acts upon the muscular fuel substance somewhat as 

 a fuse acts upon a charge of gunpowder. Even the amount 



