722 SPECIAL SENSES. 



An important point, not to be lost sight of in the study of the associated 

 action of the muscles of the globe, relates to the associated movements of the 

 two eyes. Perfect, binocular vision is possible only when impressions are 

 made upon exactly corresponding points in the retina of each eye. If one 

 eye be deviated in the horizontal plane, the points no longer correspond, and 

 there is double vision, the same as if two impressions were made upon one 

 retina; for when the impressions exactly correspond, the two retinae act 

 practically as a single organ. The same is true in deviation of the globe in 

 the vertical plane. If it be supposed, for the sake of argument, that the 

 retina is square, it is evident that a torsion, or twisting of one globe upon an 

 antero-posterior axis, must be attended with an analogous movement of the 

 other globe, in order to bring the visual rays to bear upon the corresponding 

 points ; in other words, the obliquity of the assumed square of the retina 

 must be exactly the same for the two eyes, or the coincidence of the corre- 

 sponding points would be disturbed and there would be double vision. De- 

 viation of one eye in the horizontal or the vertical plane disturbs the relation 

 of the corresponding points, and a deviation from exact coincidence of action 

 in torsion of the globes, twists, as it were, the corresponding points, so that 

 their relation is also disturbed. It is evident, therefore, that the varied move- 

 ments of the globes, by the combined action of the recti and oblique muscles, 

 must correspond for each eye, in the movements of torsion upon an antero- 

 posterior axis as well as in movements of rotation upon the horizontal or the 

 vertical axis. 



CENTRES FOR VISION. 



Experiments have been made upon the lower animals by Terrier, Munk, 

 Exner, Dalton and many others, with the object of locating in the cerebrum 

 a centre for vision. It is important, however, to compare the results of such 

 experiments with cases of cerebral lesions in the human subject. As the 

 general result of experiments, both on dogs and monkeys, and of pathological 

 observations, the present opinion is that the centres for vision are in the 

 occipital lobes. The convolutions immediately above and below the calcarine 

 fissure, on the inner surface of the cerebrum (compare Figs. 221 and 222, 

 page 605), seem to be the cerebral terminations of fibres that are continuous 

 with the optic tracts. These fibres are not crossed in the cerebrum, but the 

 conductors decussate at the optic chiasm, as they pass to the eyes. Cases 

 have been observed in the human subject, in which lesion of these parts on 

 one side has been followed by loss of vision in one lateral half of the retina 

 in either eye. This condition is called hemianopsia. In these instances the 

 blindness is confined to the temporal side of the retina corresponding to the 

 lesion and the nasal side of the retina of the opposite eye. This is called 

 lateral homonym ous hemianopsia, and this is the form which always occurs 

 in unilateral cerebral lesion. In dogs and in monkeys destruction of both 

 occipital lobes and both angular convolutions produces total and permanent 

 blindness of both eyes. 



The complete and perfect perception of visual impressions involves intel- 

 lectual action connected with the simple visual sense. An individual may 



