NEUEO-MUSCULAE MECHANISM 137 



postulated. In lateral deviation of the eyes that part of the 

 nucleus of the third nerve which presides over the internal 

 rectus of one side acts harmoniously with the sixth nerve 

 supplying the external rectus of the other side, and hence it 

 may be supposed that a directing mechanism for lateral devia- 

 tion exists possibly in the superior olive. Similarly a centre or 

 centres presiding over the movements of the eyes in a vertical 

 plain may be supposed to exist. 



The movements of the eyes involve not merely the con- 

 traction of definite muscles, but also the co-ordinated inhibition 

 of others (see p. 138). 



2. CONNECTIONS OF THE EYES WITH THE CENTRAL 

 NERVOUS SYSTEM 



From each eye the optic nerve extends backwards and 

 inwards to join the other optic nerve at the chiasma. 



A partial crossing of the fibres takes place in the chiasma, the 

 extent of decussation varying in different animals and being 

 fairly extensive in the horse. From the chiasma the two optic 

 tracts pass upwards round the crura cerebri to end in two 

 divisions 



1. A posterior division passing to the anterior corpora 

 quadrigemina on the same side (fig. 67, A.C.Q.). 



2. An anterior division running to the geniculate body on 

 the posterior aspect of the thalamus opticus (fig. 73, Op. Th.}. 



The fibres of the posterior termination of the optic tract 

 end in synapses with neurons in the corpora quadrigemina, 

 and the fibres of these neurons pass downwards and control 

 the oculo-motor mechanism already described (fig. 66, p. 136). 



The fibres of the anterior division make synapses with other 

 neurons in the posterior part of the thalamus, and these 

 neurons send their fibres backwards to the occipital lobe of 

 the brain, where they connect with the cortical neurons (fig. 

 67, Occ., p. 138). 



3. THE VISUAL CENTRE 



A response to stimulation on the part of the neurons in the 

 occipital lobe of the brain (p. 201) is the physical basis of 



