628 



PHYSIOLOGY 



The light which falls into the eye forms an image of external objects 

 which extends over the whole of the retina. The sensations 

 excited by the stimulation of the periphery of the retina are much 

 more indistinct than those excited by the image on the central spot. 

 The appreciation of external objects, by means of the image they 

 throw on the external parts of the retina, is spoken of as indirect 

 vision in contradistinction to direct vision, which implies fixation of 

 the object and the formation of an image of it on the fovea centralis. 

 The whole extent of the objects which we can see by direct and indirect 



vision is spoken of as the 

 visual field. In order to de- 

 termine the visual field we 

 make use of a perimeter 

 (Fig. 288). 



This instrument consists of a 

 band of metal forming the arc of 

 a circle of about 35 cm. radius. 

 At one" end this arc is fastened to a 

 pillar, and can be turned through 

 the axis passing through the pillar 

 so as to lie in various meridians. 

 At the centre of the circle is another 

 pillar, which provides a chin -rest, 

 so arranged that ' the eye of the 

 observed person lies exactly at the 

 centre of the circle at the top of 

 the pillar. At the point round 

 which the arc rotates is a small 

 white disc. In using the instru- 

 ment the person, whose field of 

 vision is to be determined, places 

 his eye at the top of the pillar and 

 gazes fixedly at the white disc ; 



another small white disc is then moved along the curved arc and the point 

 noted at which it is no longer visible, while the observed person is gazing 

 fixedly at the white disc on the axis of rotation. The arc is then moved 20 

 and the same experiment carried out, and this is continued until the limit has 

 been determined in every meridian of the visual field. The rotating arc is 

 graduated, the graduations showing the visual angles subtended by any portion 

 of the arc. As the readings are made they are marked down on a chart, such 

 as that shown in Fig. 289, so that finally a graphic representation of the 

 visual field is obtained. The visual field is more extensive on the outer than on 

 the nasal side of the eye, the latter being contracted by the cutting off of some 

 of the rays falling on the outer side of the retina by means of the nose. 



Although stimulation of the peripheral parts of the retina does not 

 give us much idea of the nature of the things we are looking at, yet 

 it is of great importance in informing us of the relation of the object, 

 which is the immediate point of attention, to its surroundings. It 

 therefore plays a great part in regulating the movements of the body. 



FIG. 288. Priestley Smith's perimeter. 



