VISION IN THE MOUSE 585 



my hand at any time. Further, the fact that the head was moved 

 would make the view that fear inhibited the eye-movements un- 

 tenable. 



Such slight eye-movements as do occur seem to be rather for 

 the purpose of getting more light from the general direction of the 

 object than for getting a clearer image. There appears to be 

 nothing in the vision of the mouse which compares with fixation 

 in the human being. If the image of the object is cast upon any 

 part of the retina, all the conditions are fulfilled which make 

 vision' useful to the mouse as a protective sense. 



On the third requirement for binocular vision, the dividing 

 and crossing of the optic fibres, positive statements can not be 

 made from a mere study of the anatomical structure. The chiasm 

 exists in all vertebrates, while in mammals, birds and to some 

 extent, in reptiles the nerves unite, so that each is made up of 

 fibres from both roots. In fishes they cross without uniting. 

 Whether fibres from corresponding points in the retinae unite so 

 that their excitations are carried to the same neural channels in 

 the brain is practically undiscoverable, but the fact that there is 

 any union at all rather than total decussation, indicates an 

 intimate degree of relation between the two eyes. 



Harris, 2 who worked by the method of degeneration upon the 

 optic fibres of the lizard, ventures the statement that decussa- 

 tion in rats and mice is complete. More satisfactory knowledge 

 on this point would be gained from the cumulative evidence of 

 the degeneration method and the method of myelogenesis as used 

 by von Gudden. The amount of mingling of optic fibres would not 

 be great considering the fact that only small portions of the 

 retinae can receive images from the same object. Cajal 3 working 

 by the method of degeneration upon the optic chiasm of the rab- 

 bit and the mouse, has in fact shown that the crossing of the fibres 

 is not complete. 



2 Harris, W. Binocular and stereoscopic vision in man and the other verte- 

 brates, with its relation to the decussation of the optic nerves, the ocular move- 

 ments and the pupil light-reflex. Brain, part cv, p. 107. 1904. 



3 Ramon y Cajal, S. Textura del sistema nervioso del hombre y de los verde- 

 brados, vol. 2, p. 652, Madrid, 1904. 



