564 



PHYSIOLOGY 



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 nated 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 



XI 



vn 



FIG. 291. Perimeter chart showing the field of vision in a normal (right) eye. 



the readings are made they are marked down on a chart, such as that shown in Fig. 

 291, 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 0& 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. Its importance will 

 be at once appreciated if one eye be closed and the stimulation of the peri- 

 pheral parts of the retina in the other eye be excluded by allowing this eye 

 only to look through a tube. Although we can then see the objects to which 

 we direct our gaze perfectly distinctly, we find that on trying to move towards 

 any given object our movements are uncertain and misdirected. 



CHEMICAL AND PHYSICAL CHANGES IN THE RETINA 

 When light falls on the retina chemical and physical changes take place ; 

 these either originate or accompany the transmutation of the ether vibrations 

 into the nerve impulses, which ascend the optic nerve. If a frog that has been 



