COORDINATION 



79 



then obtain a plan like that shown in Fig. 44, and such, in 

 principle at least, represents the manner in which the afferent 

 neurone is brought into relation with efferent neurones. 



Afferent and efferent fibers enter and leave portions of the 

 brain in much the same way, although the separation into 

 ventral and dorsal roots is not obvious. We may there- 

 fore take the above scheme as typical of the relation be- 

 tween these two kinds of neurones those of the brain as 

 well as those of the cord. 



10. Application of these facts 

 of structure in the explanation 

 of purposeful and coordinated 

 action. The diagram in Fig. 45 

 readily explains why the sud- 

 den appearance of an object in 

 front of the eye causes us to 

 wink and not cough ; that is to 

 say, it explains the purposeful 

 character of this so-called reflex 

 action. The formation of the FIG. 45. Diagram of the nervous 

 image of the object on the mechanism by which a wink is pro- 

 duced by the sudden appearance of 

 retina, a sense organ, starts an object in front of the eye 



impulses along the fibers of r , afferent neurone of the optic nerve ; 



the afferent optic nerve ; these m > m/ > m "> m '"> efferent neurones to 



. . the muscles of the eyelid 



fibers extend into the brain, 



and their synapses end around and stimulate those efferent 

 nerve cells which stimulate the muscles of the eyelid. The 

 action is purposeful because the fibers of the optic nerve 

 end around these cells and not around those which, for 

 example, innervate * the muscles which open the mouth or 

 flex the finger (Fig. 45). 



Our diagram also gives the basis of coordination the 

 combination of the work of different muscle fibers in orderly 

 harmonious action. The system of collaterals on the central 

 i That is, supply with nerve fibers. 



