CHAP, in.] SIGHT. 39 



It is desirable to remember one important difference as to the 

 behaviour of the pupil which obtains between man and some of 

 the higher mammals on the one hand, and the lower mammals as 

 well as other vertebrates on the other. In the former, the pupil- 

 constricting nervous mechanisms of the two eyes are not completely 

 independent ; there is a functional communion between the two 

 sides, so that when one retina is stimulated both pupils con- 

 tract, and indeed, in man, as a rule, contract equally. Hence, 

 when a change in the pupil of one eye is brought about by some 

 means other than the one we are now considering, the pupil 

 of the other eye is affected ; when for instance one pupil is 

 dilated with atropin, the larger amount of light thus admitted 

 into that eye causes a narrowing of the pupil of the other eye, and 

 thus increases the difference between the pupils of the two eyes. 

 In the lower mammals and other vertebrates, the mechanisms in 

 question are independent, stimulation of one retina produces no 

 effect on the pupil of the other eye. This difference seems 

 dependent on the character of the optic decussation, which, in 

 turn, as we have seen ( 667), is connected with the presence or 

 absence of binocular vision. In man, with considerable binocular 

 vision, the decussation is partial only, a considerable part of the 

 optic nerve passes into the optic tract of the same side (Fig. 133) ; 

 in the dog, with more limited binocular vision, a much smaller 

 proportion of the optic nerve takes this course ; and in the lower 

 mammals, and other vertebrates which possess no binocular vision, 

 the crossing is complete, the whole optic nerve of the one side 

 passes into the optic tract of the opposite side, though the bundles 

 of fibres may be interwoven at the decussation in various ways in 

 different animals. If we reject the view mentioned above, that 

 the optic fibres subserving the pupil mechanism leave the optic 

 nerve at the decussation and suppose that these fibres pass with 

 the other optic fibres into the optic tract, we must conclude that 

 in such an animal as the frog, or bird, in which stimulation of the 

 retina produces constriction of the pupil of the same eye and of 

 that alone, a double crossing of impulses must take place ; the 

 afferent impulses of the pupil-constrictor mechanism must cross 

 over to the opposite optic tract, and so reach the opposite centre, 

 and the efferent impulses which these generate in the centre must 

 cross back again to reach the pupil of the eye whose retina was 

 stimulated. But such a conclusion is opposed to the results 

 which have been obtained on the dog, in which, as we have just 

 said, the decussation is partial. In this animal, after division of 

 the floor of the third ventricle and aqueduct lengthways in the 

 median line, not only does unilateral direct stimulation of the 

 pupil-constrictor centre, or at least of the region of the nucleus of 

 the third nerve, produce constriction in the pupil of the same side 

 alone, but it is stated that, under these circumstances, reflex 

 constriction of the pupil may be obtained, on either side, by 



