82 THE HUMAN MECHANISM 



the two cases, but the source of stimulation of the efferent 

 neurones must be different. 



In later chapters of this book we shall bring forward 

 evidence to show that the exercise of the will (volition) 

 requires the cooperation of the highest portion of the brain 

 or cerebrum. Nerve cells in the gray matter of the cerebrum 

 send off axons whieh pass downward to those portions of the 

 brain and spinal cord from which the motor or efferent neu- 

 rones arise ; with the neurones of these nerves they make ex- 

 actly the same kind of connections (collaterals and synapses) 

 as are made by the afferent fibers from the retina which excite 

 the reflex (see Fig. 46, in which b is the cerebral neurone). 



The collaterals and synapses of the cerebral neurone 

 (which, it will be observed, is entirely confined to the cen- 

 tral nervous system) simply duplicate those of the afferent 

 neurone ; hence the two neurones produce the same result. 



There is, however, still a third way in which winking may 

 be stimulated. When the cornea of the eye begins to dry, a 

 reflex wink spreads tears over the eyeball. In this case we 

 have to deal with a second reflex, the afferent neurones being 

 not those in the optic nerve, but those in what is known as 

 the trigeminal, the sensory nerve of the cornea. Our scheme 

 thus becomes that shown in Fig. 47. 



13. The " master " neurone. The multiplication of collat- 

 erals and arborizations which this scheme involves would 

 seem to be largely avoided by the presence of a third neu- 

 rone between those which stimulate the action and the effer- 

 ent neurones which directly act on the muscles (Fig. 48). 



In this way, when a wink is produced, whether from the 

 cerebrum or from the retina or from the cornea, the single 

 cell a is stimulated, and this in turn stimulates the groups 

 of efferent neurones which immediately innervate the mus- 

 cles of the eyelids. Many of the nerve fibers of the cord and 

 brain belong to neurones which perform the same function 

 as that attributed to the cell a in our diagram. They are 



