162 VETERINARY PHYSIOLOGY 



exists, possibly in the superior olive (fig. 80). Similarly a 

 centre or centres presiding over the movements of the eyes 

 in a vertical plane may be supposed to exist. 



6. Disturbances of the Neuro-Muscular Mechanism— Paralysis 

 of these muscles and of the nerves supplying them leads 

 to a loss of the co-ordinated movements of the two eyes, 

 with the result that the optic axes are no longer parallel 

 and squint is produced. As a result of this, corresponding 

 points on the two retinae are not stimulated by rays from the 

 same object, and double vision, diplopia, results in men and 

 apes. 



III. CONNECTIONS OF THE EYES WITH THE CENTRAL 



NERVOUS SYSTEM. 



1. THE OPTIC NERVES AND OPTIC TRACTS. 



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 colliculus 

 of the tectum on the same side (fig. 81, A.G.Q.). 



2. An anterior division running to the external geniculate 

 body on the posterior aspect of the thalamus opticus (fig. 

 81, 02y.Th.). 



In most of the lower animals a complete decussation of 

 the optic fibres takes place, so that the nerve fibres from 

 the left eye go to the right side of the brain, and vice 

 versa. But in man and apes a partial crossing of the fibres 

 takes place in the chiasma — fibres from the middle and 

 internal part of the retina decussating, those from the 

 outer part remaining on the same side. For this reason, 

 section of the right optic tract leads to partial blind- 

 ness of both retinae — on the outer part of the right eye 

 and on the inner and middle part of the left eye, so that 

 objects on the left side of the field of vision are not seen. 



