HUMAN PHYSIOLOGY 



DBG is larger than CBA. The angle ABC is the measure of 

 the vertical field of vision, and it will be observed how it is 

 constricted by the eyebrow. The vertical angle amounts to 

 about 130 ; 60 in the upper field and 70 in the lower field. 



The horizontal field worked out in the same way gives 

 about 150, of which no less than 90 are on the outer side 

 and only 60 on the inner. 



Of all parts of the retina the central spot is the most 

 sensitive to differences of illumination. 



(3) The layer of the retina capable 

 of stimulation is the layer of rods and 

 cones. This is proved by the experi- 

 ment of Purkinje's images. It depends 

 upon the fact that if a ray of light is 

 thrown through the sclerotic coat of 

 the eye the shadow of the blood-vessels 

 stimulates a subjacent layer (Fig. 56, c), 

 and these vessels appear as a series of 

 wriggling lines on the surface looked 

 at. If the light is moved the lines 

 seem to move, and, by resolving the 

 triangles, it is possible to calculate the 



FIG. 56.-To show that the distance behind the vessels of the part 

 stimulated, and this distance is found 

 to correspond to the thickness of the 

 retina. (Experiment.) 



(4) Modes of Stimulation. The 

 rods and cones are generally stimu- 

 lated by the ethereal light vibration, 

 but they may be stimulated by 



mechanical violence or by sudden changes in an electric 

 current. But, however stimulated, the kind of sensation is 

 always of the same kind a visual sensation. (See p. 104.) 



(5) Of the nature of the changes in the retina when 

 stimulated we know little. But we know 



1st. That under the influence of light the cells of the 

 tapetum nigrum expand forward between the rods and cones. 



2nd. That a purple pigment which exists in the outer 

 segment of the rods is bleached. Even although there is 

 no purple in the cones, which alone occupy the sensitive 



hindmost layer of the re- 

 tina is stimulated. (Pur- 

 kinje's Images.} a, source 

 of light ; b, blood - vessel 

 of retina; c, shadow of 

 vessel on rods and cones ; 

 d, image of shadow men- 

 tally projected on to the 

 wall. 



