THE REGULATING MECHANISM 205 



the nasal side of the left, while the left corpus 

 receives fibres from the left temporal and the 

 right nasal side. This secures single vision 

 with two eyes, if the image falls on correspond- 

 ing parts of the two retinae, as, for example, if 

 it falls on the right temporal and left nasal. 

 But if one image was formed on both temporal 

 or both nasal halves of the retina, there would 

 be double vision, as in squinting. 



118. The optic thalami are sensory centres, 

 receiving impulses from below, and from them 

 impulses pass upwards to the cerebrum, where 

 they excite consciousness. They (a) may act, 

 however, as reflex mechanisms in conjunction 

 with the two masses immediately in front of 

 them, the corpora striata (b). These are motor 

 centres, receiving impulses from the motor 

 regions of the cortex of the cerebrum (c). 

 If a and b act together, without the influence 

 of c, there may be complicated reflex move- 

 ments, such as occur in the walking of the 

 somnambulist, or the unconscious perform- 

 ance of complicated movements. 



119. The highest of all centres is the 

 cerebrum, consisting of two hemispheres, show- 



