550 



PHYSIOLOGY 



The values obtained by means of this instrument are shown in Fig. 280, 

 which give 5 a typical curve for the right eye. The shaded area on the 

 left of the diagram is due to the obstruction of the eyebrow, nose and cheek 

 of the patient. The visual field on the outer side will be seen to extend 

 actually 14 degrees beyond the right angle. This result, which at first sight 

 appears to be impossible, is in fact due to the considerable refraction of the 



XI 



VH 



l-'ji ;. L'SO. Field of vision for white and colours of a normal right eye as obtained liy 

 the perimeter. (HABTRIDGE.) 



light rays that occurs at the extreme edge of the cornea. The direction of 

 tin- beam ol light entering the eye under these circumstances is shown in 

 Fig. 281 X below. 



It will be observed that even when looking straight in front, a man 

 can see to a considerable angle behind himself. By deviating the eyes only 

 slightly to either side, this angle can be increased to 40 degrees in the average 

 case as shown at Fig. 281 Z. This ability to see to a considerable extent 

 behind him is due to the narrowness of the head between the 

 frontal processes and zygomatic bones. In those animals in which 

 the eyes are placed on the sides of the head, and the visual axes are diainet ri 

 cally opposite to one another, the visual fields will actually overlap a short 

 distance From the head, so that there will be no direction from which an 

 enemy can attack without being observed (Fig. 281 Y). 



