84 



THE HUMAN MECHANISM 



spreading tears over the surface of the eyeball and, to be 

 effective, must be accompanied by a secretion of tears. We 

 may suppose that this is accomplished, as in the diagram, 

 by the afferent neurone (c) from the cornea stimulating two 



master neurones, one of 

 which (m w ) produces the 

 wink, while the other 

 (nt) stimulates the tear 

 glands to secrete. 



The wink from the 

 retina, on the other hand, 

 has the entirely different 

 purpose of preventing 

 the contact of foreign 

 objects with the cornea. 

 For this purpose tears 

 are not necessary and 

 they are not secreted. 

 But at times this wink 

 is accompanied by a 

 sudden starting back of 

 the body as a whole 

 to avoid the threatened 

 danger. In this case we 

 may suppose that the 

 afferent neurone from 

 the retina connects with 

 the master neurones m w , 

 for winking, and m sb , for 

 starting back, but that 

 this afferent neurone 

 does not connect with m\ for the secretion of tears. 



Finally, the volitional neurones v sb and v w , which pass from 

 the cerebrum to their appropriate master neurones, call forth 

 these actions of starting back or winking as separate acts. 



FIG. 49. Coordinations involved in the com- 

 bination of the wink with other actions 



The afferent neurones r, from the retina, and 

 c, from the cornea, connect with different com- 

 binations of efferent neurones as explained in 

 the text. Efferent master neurones are shown 

 as follows : m w , for winking ; m f , for secretion 

 of tears ; m' b , for starting back. Neurones con- 

 cerned in volitional actions are v* b , for starting 

 back, and v w , for winking 



