THE SENSORI-MOTOR SYSTEM 



29 



goes to the lifting muscles of the eyelids, and the latter goes 

 to the closing muscles. The same event in the centre gives rise 

 to both impulses simultaneously, and that travelling along the 

 oculo-motor nerve causes the opening muscles to relax, while 

 that which travels along the branch of the facial causes the 

 closing muscles to contract. The eyelids thus close. Im- 

 mediately afterwards the series of events is reversed because 

 of a second pair of efferent impulses; the opening muscles now 

 contract, and the closing ones relax, and so the eyelids open. 



Representing this series of action in a very schematic way, we 

 get the diagrams given on pp. 28 and 29. 



\jiefma. 



[ot>er,'mg 

 '^ muscHe) 



Fig. 11. 



-Scheme of the Connections between the Retina, Brain, 

 AND Eyelids used in the Act of Winking. 



According to the figure, there is only one proximal set of den- 

 drites in the retina ; in reality, there are three (as is represented in 

 Fig. 30, p. 107). Further, only one centre is shown, but really 

 the optic nerve ends in three centres in the mid-braui (see Fig. 32, 

 p. 115). From these centres other neurones make connection 

 with the oculo-motor centre, and from the latter another set 

 of neurones pass out along the oculo-motor nerves to the opening 

 muscles, while yet another traverses the nucleus (or centre) of 

 the facial nerve, and go out through the latter to the closing 

 muscles of the eyelids. 



In any activity of the sensori-motor system, then, a rather 

 complicated mechanism is involved. Some change occurring in 



