NERVE 165 



defined as we pass from the simpler mammals, with separate 

 vision for the two eyes, to the primates with combined vision 

 with the two eyes. 



The evidence that it is this part of the brain which must 

 be stimulated in order that visual sensations may be experi- 

 enced is convincing. But it must be remembered that there 

 are lower arcs connecting with the oculo-motor mechanism 

 from the optic nerves — (1) through the thalamus and red 

 nucleus, and (2) through the anterior part of the tectum, and 

 that stimulation of the eye may lead to movements of the 

 body and of the eye through these without consciousness 

 being involved. This is seen in the reaction of the pupil to 

 light and in the reflex movements of the eyes in response to 

 unilateral auditory stimulation. 



The centre is united by a strong band through the middle 

 peduncle with the cerebellum (see p. 130), and in the higher 

 apes by another band of fibres with an area in the frontal 

 region, concerned with the movements of the eyes, and this 

 is again connected with the cerebellum (fig. 58). 



Usually the visual area is stimulated by changes passing 

 up the series of neurons from the retina, but it may also be 

 stimulated directly, as is sometimes seen in the early part of 

 an epileptic fit, or by the previous action of other chains of 

 neurons, as in dreaming. 



The strength of the sensation varies with the strength of 

 the stimulus, and the smallest difference of sensation which 

 can be appreciated is a constant factor of the degree of 

 stimulation. 



The sensation lasts longer than the stimulus, and thus, 

 if a series of stimuli follow one another at sufficiently rapid 

 intervals, a fusion of sensations is produced. If a wheel 

 rotating slowly is looked at, the individual spokes are seen, 

 but when it is going more rapidly, the appearance of a 

 continuous surface is presented. When the light is dim, 

 this fusion takes place more readily than when the light is 

 bright. 



The visual centre of each side must be rescarded as a 

 chart of the opposite field of vision, each part corresponding 



