PROPERTIES OF THE RETINA. 339 



estimated for the bottom of the depression, 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 nodal point of the eye sub- 

 tend an angle of 1 degree to 1.5 degrees; and, therefore, all objects 

 in the external world around the line of sight which lie wdthin this 

 visual angle 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 (hemianopia) in the retinas on the 

 same side, — that is, lesions in the right occipital lobe cause blind- 

 ness 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. 206) ; 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 its special 

 functions in vision the fovea centralis possesses a peculiar struc- 

 ture. It forms a shallow depression in the center of the retina 

 described by some authors as elliptical, by others as circular in out- 

 line. In the center of the fovea lies a smaller, very shallow depres- 

 sion spoken of as the foveola. The diameter of the fovea, as stated 

 above; is estimated differently by different authors. While meas- 

 urements on preserved specimens give the diameter as 0.2 to 

 0.4 mm., ophthalmoscopic examination seems to indicate that in 

 the fresh state it may be larger. According to Fritsch,* the fundus, 

 reckoned from the point at which the depression begins, has a diam- 

 eter of 0.5 to 0.75 mm. Within the fovea cones only are present, 

 and these cones are longer and more slender (diameter, 0.002 mm.) 

 than in the rest of the retina. Moreover, the thickness of the retina 

 is much reduced in the fovea, whence arises the depression. At 

 this point the cones are practically exposed directly to the light, 

 whereas in other parts the light must penetrate the other layers 

 before reaching the rods and cones. Lying around the fovea is an 

 area about 6 mm. in diameter, of a yellowish color, and hence 

 known as the macula lutea. The portion of the visual field falling 

 upon this area, in a fixed position of the eye, is sometimes called the 

 macular field. According to some observers f the yellow color of 

 the macula is due to post-mortem changes. 



Visual Acuity.^ — The distinctness of vision or the resolving 

 power of the eye varies greatly in different parts of the retina. 

 It may be measured for the fovea by bringing two fine lines closer 

 and closer together until the eye is unable to see thein as two dis- 

 tinct objects. Measured in this way, it is usually stated that when 

 the distance between the lines subtends an angle of 1 minute 

 (60 seconds) at the eye, the limit of resolvability is reached. This 



♦Fritsch, "Sitzungsberichte d. konig. Akad. d. Wiss.," Berlin, 1900. 



t Siven, " Skandinavisches Archiv f. Physiol.," 1905, 17, 306. 



