138 



ESSENTIALS OF PHYSIOLOGY. 



In man vision is binocular, and the eyeballs always move together 

 in such a way that the image of the object looked at falls on the fovea 

 of each eye. If the object is a distant one, the visual axes are parallel ; 

 if the object is a near one, there is convergence of the visual axes. In 

 either case objects which are not in the direct line of vision fall on 

 " corresponding points " of the two retinse. If the mechanism for 

 combined movements fails for any reason, the images of external 

 objects are not formed on corresponding points, and double vision, or 

 diplopia, results. 



THE FIELD OF VISION. 



When an object is looked at its image is formed on the fovea, 

 and it is seen distinctly. This is known as "direct vision." At the 



same time, surrounding 

 objects are focussed on 

 the retina outside the 

 fovea and are seen less 

 distinctly. This is 

 known as " indirect 

 vision." The extent of 

 the outer world in- 

 cluded in both direct 

 and indirect vision con- 

 stitutes the visual field, 

 and is measured by 

 means of an instrument 

 called the perimeter. A 

 simple form of this con- 

 sists of a graduated arc 



FIG. 43. Perimetric chart for right eye, showing fields which can be moved 

 for white, blue (and yellow), red, and green. into any mer idian, and 

 ( After Howell.) . / 



which is provided with 



a white spot at its axis. The subject closes one eye, and with the other 

 gazes steadily at the white spot. A white or coloured object is then 

 moved from the extreme end of the arc until it comes into the field of 

 vision, when its position is recorded on a chart. This is repeated for 

 other meridians, and then the recorded points are connected on the 

 chart by lines, thus giving a map of the field of vision. The field for a 

 white object is larger than that for a coloured object, and, of the 

 primary colours, blue and-yellow have the largest field and green the 

 smallest, while red is intermediate (fig. 43). 



The field for white extends to 90 on the temporal side, about 80* 



