FORM-SENSE 53 



of a specially slender variety. Ramon y Cajal showed 

 that each of the central cones is connected with but one 

 bipolar cell and but one ganglion cell, thus differing from 

 the end-organs in the more peripheral parts of the retina, 

 each of which is connected with several bipolar and several 

 ganglion cells. 



The acuity of form-sense in man is most intense at the 

 fovea and rapidly decreases on proceeding from it toward 

 the periphery; thus it has been approximately estimated 

 that 5 eccentric from the center of the fovea the acuity is 



\j 



reduced to J and 20 eccentric to ^V- 



In the development of the human retina an area centralis 

 is recognizable before the fovea centralis makes its appear- 

 ance. According to von Hippel, 24 in the central area of the 

 retina at birth all the layers are recognizable, though in the 

 situation where the fovea ultimately forms, the cells of the 

 ganglion cell-layer and of the inner nuclear layer are some- 

 what thin and spaced out. By the end of the fourth week 

 a depression is formed, but the full development of the fovea 

 is not completed until several months have elapsed. This 

 gradual unfolding of the fovea corresponds with what has 

 been observed regarding the development of an infant's 

 power of fixation. Worth 25 has observed that the pupil 

 contracts in a newborn infant if a light is suddenly flashed 

 on to its eye in the dark, and that the child will try to fix 

 it for an instant, as by a reflex act, showing that some pre- 

 ponderance in power at the macula is innate. He says: 

 "At the end of two or four weeks most infants will fix a 

 light steadily for several seconds at a time with one or other 

 eye, but will not converge both visual axes accurately in 

 looking at a near object. It is not until five or six weeks 

 that binocular fixation manifests itself.' 



