55o 



A MANUAL OF VETERINARY PHYSIOLOGY 



In other cases a large proportion of the fibres do not cross in the 

 chiasma, so that if these are proceeding from the right eye, 

 they pass to the right brain. This condition exists in man and 

 those animals with the eyes centrally placed in which binocular 

 stereoscopic vision only exists. Between these comes another 

 group, where a certain, but small, proportion of the fibres are 

 direct, but the bulk decussate ; and such we saw was the con- 

 dition in the horse, in which it is stated one-sixth of the fibres 

 only are direct. In man vision is binocular and stereoscopic, 

 and the conditions for stereoscopic vision are that the visual im- 

 pressions from the left halves of the two retinae shall be superimposed 

 on the left occipital cortex, while those from the right halves are 

 similarly impressed on the right visuo-sensory area. An animal 

 may have binocular vision ; it is met with in some birds and reptiles, 

 but, nevertheless, it is not stereoscopic, for the reason that there are 

 no direct fibres in the optic nerve. In the cat and tiger there is, 

 as in man, true stereoscopic vision, the visual axes being roughly 

 parallel, and a sufficient number of direct fibres passing from the 

 retina to the same side of the brain. Harris* considers that the 

 presence of binocular or monocular vision in animals depends upon 

 their feeding habits. Carnivora, in order to be quick and sure, 

 have their eyes set well forward ; in the herbivora, whose food does 

 not run away from them, the eyes are 

 laterally placed. In carnivorous birds, such 

 as the owl, the presence of binocular vision 

 (not stereoscopic) assists the accuracy of 

 sight in twilight. Mottf regards the pro- 

 duction of binocular stereoscopic vision as 

 depending on the specialisation of the fore- 

 limbs for prehension of prey ; this would 

 be less effective in the absence of stereo- 

 scopic vision, and he regards it as the deter- 

 mining factor rather than the nocturnal 

 habits of animals. 



When a horse directs both eyes to the 

 front (Fig. 167) he produces a well- 

 marked double internal squint, and is 

 then capable of binocular vision. The 

 eyes are rotated inwards and slightly 

 upwards by the combined action of the 

 inferior oblique and internal rectus ; the 

 pupils are not perfectly horizontal, but 

 nearly so, and the pupillary opening is 

 brought so far to the front that the 

 Fig. 167.— The Position i nner segment of the cornea and iris 

 of the Head and Eyes 1 j« u j/l ±-u 



in Binocular Vision. entirely disappears beneath the inner 

 canthus. In no other position than 

 this has the horse binocular and presumably stereoscopic vision. 

 Animals with their eyes situated on the lateral side of the head 

 are capable of exercising monocular vision for all objects placed 

 to one side of them, and even behind them. There is good 

 * Op. cit., p. 535 t Op. cit., p. 541. 



