314 



THE SPECIAL SENSES. 



retinal image from the fovea. In using our eyes, therefore, we keep 

 them continually in motion so as to bring each object, as we pay 

 especial attention to it, in the field of central vision. The line 

 from the fovea to the point looked at is designated as the line of 

 sight. The area of the fovea is quite small. The measurements 

 given by different observers vary somewhat, especially as in some 

 cases the measurements are estimated for the bottom of the de- 

 pression, the fundus, and in others for the diameter from edge to 

 edge. The average diameter is usually given as lying between 0.3 

 and 0.4 mm. Lines drawn from the ends of this diameter to the 



Fig. 135. Perimeter chart to show the field of vision for a right eye when kept in a fixed 



position. 



nodal point of the eye subtend an angle of 1 degree to 1.5 degrees; 

 and therefore all objects in the external world around the line of 

 sight whose visual angle is within this limit are comprised in the 

 central field of vision, and their retinal images fall upon the fovea. 

 Unilateral lesions of one occipital lobe cause half-blindness (hemi- 

 opia) in the retinas on the same side, that is, lesions in the right 

 occipital lobe cause blindness of the right halves of the retinas, while 

 injuries to the left occipital lobes are accompanied by loss of vision 

 on the left sides of the retinas (see p. 193); but such unilateral 

 lesions, it is stated, do not involve the central field of vision only 

 the peripheral portion of the field is affected. In connection with 



