VISION IN THE MOUSE 587 



the retina, where the lines of sight which converge on the object 

 terminate. 



Although it is true that corresponding points function most 

 accurately in the region of the central spot, as is shown by the 

 difficulties attending the experimental determination of the horop- 

 ter, yet it need not be true that they owe their existence to the 

 presence of a central spot. The existence of corresponding points 

 is quite possible in a retina, all portions of which are similarly 

 organized. 



The mouse, which has no fovea, might have certain portions of 

 the eye adapted for binocular vision. These are the extreme pos- 

 terior areas of the retina, which correspond to the temporal seg- 

 ments in the human being. It is here that images from one object 

 can fall on both retinae and, therefore, here corresponding points 

 must have been developed, otherwise the animal would perceive 

 objects double. 



The conditions here differ from those obtaining in the human 

 being only in degree. In the latter a considerable portion of 

 the nasal area of each retina can not function in binocular vision 

 on account of the prominence of the bridgeof the nose, and there- 

 fore a point in this region can have no point in the temporal por- 

 tion of the other retina corresponding to it. In the mouse this 

 area is much more extensive. If, for convenience, we call this 

 area the monocular area, as distinguished from that in which 

 corresponding points exist — the binocular area, then in an animal 

 like the mouse the centre of the retina lies in this monocular area. 



If there were a point of clearest vision near the center of the eye, 

 it would be merely a fixation point which might function in len- 

 ticular accommodation. However there is no structural sign of a 

 fovea in this region and obviously such a point in the monocular 

 area could not function in convergence. We must conclude that 

 the optical axis of the mouse's ej^e has no functional significance. 



If we expected to find a fovea which serves the mouse as ours 

 serves us, we would look for it in the extreme posterior portion of 

 the retina, in the binocular area. 



I made a number of observations upon squirrels in connection 

 with this problem. In the squirrel the position of the eyes is some- 



