VISION IN THE MOUSE 581 



mice the number of right choices being 47 per cent of the number 

 of trials, while for the albino mice it is 53 per cent — it was thought 

 that this head movement might be serviceable to the animals, as 

 giving several points of view of the object, each from a different 

 angle, thus possibly rendering a perception of depth easier. 



Following this idea, a record was kept of the number of times 

 the head movement was observed and this was compared with the 

 right choices. I present as typical the results in the case of one 

 mouse, Kh. This animal made 14 right choices and 16 wrong. 

 Of the right choices the head movement was observed in 8 and not 

 observed in 6, while of the wrong choices there was head move- 

 ment in 1 and no head movement in 15. 



Conclusions: Judging by the number of errors, we may con- 

 clude that the mice do not make use of visual perception of depth. 

 If they have the anatomical equipment necessary for the percep- 

 tion of depth, their important muscular sense controls their 

 actions, making them take the same course they took on the pre- 

 ceding occasion. 



II. BINOCULAR VISION 



The question of binocular vision in the mouse suggested itself 

 in connection with the investigation into the perception of depth, 

 and an attempt was made to find how far the structural condi- 

 tions are fulfilled which would make it possible. 



The conditions which must exist in order that binocular vision 

 in the psychological sense may be present are : 



(1) It is necessary that the eyes be so situated in the head as 

 to have a portion of the field of vision common to each. 



(2) There must be consensual movements of the eyes. The lines 

 of sight of the two eyes must be capable of moving approximately 

 parallel to one another so that the images of an object may fall 

 on corresponding points of the two retinae. 



(3) There must be a chiasm of the optic nerve and a portion of 

 the fibres from one eye must mingle with a portion of those from 

 the other, that is, there must not be a total decussation of optic- 

 fibres at the chiasm. 



THE JOURNAL < '!•" « OMPARATIVE NECROLOGY AND PSYI HOLOGY, VOL. 20, NO. <>. 



