884 



THE SENSE OF SIGHT 



inhibitor processes going on in the spinal cord during reciprocal inner- 

 vation. It has been pointed out by Sherrington that the extensor or 

 stepping reflex may be inhibited by evoking the flexor reflex. In an 

 analogous manner it is supposed here that the excitation of one part 

 of the retina prevents similar processes from developing in neighboring 

 regions or in the neurons innervating them. 



The Sensibility of the Retina to Colors. — It will be remembered 

 that the perimeter is used to map out the visual field for ordinary 

 objects. It may also be employed for studying the distribution of the 

 color sense by simply replacing the small white disc by discs of 

 different colored paper. By bringing the latter into the line of 

 vision along the different meridians of the eye, it will be found that 

 the extreme outer zone of the retina is color blind and perceives only 



081 



Fig. 482. — Perimeter Chart Indicating the Average Fields of Vision for Blue, Red, 

 AND Green Compared with White (Gray). (Howell.) 



objects as such. Somewhat nearer the centel^ of the visual field, we 

 perceive first blue, then red, and lastly, green. Consequently, the 

 retina may be divided into three concentric color zones, namely, a 

 peripheral one for black and white, an intermediate one for yellow and 

 blue, and a central one for red and green. It is to be noted, however, 

 that these zones are rarely identical in the retinae of different individu- 

 als and may even present marked irregularities in one and the same 

 person. But, many of these variations are referable to differences in 

 the relative saturation of the colors employed for this test.^ Most 



1 Baird, The color sensibility of the peripheral retina, Publ. Carnegie Insti- 

 tution, No. 29, 1905. 



