SENSE OF SIGHT. 623 



anterior surface by from 0.20 to 0.30 millimetre's distance. It is, there- 

 fore, one or both of the posterior layers, namely, that of the rods and 

 cones, and that of the nuclei immediately within it, which contain the 

 sensitive elements of the retina, and in which the luminous rays produce 

 their effect. This conclusion is rendered still more certain by the fact 

 that in the fovea central is, the point of most distinct vision, hereafter 

 to be described, the two external layers of the retina are the only ones 

 present. 



Macula Lutea and Point of Distinct Vision. The macula lutea, or 

 yellow spot of the retina, is an oval spot, measuring about 2 millimetres 

 in its horizontal diameter, situated between 2 and 2.5 millimetres out- 

 side the entrance of the optic nerve. According to Helmholtz, it is 

 placed a very little beyond the middle of the fundus of the eyeball, 

 toward the temporal side. It is distinguished from the remainder of 

 the retina by its yellow tinge, which depends upon the presence of a 

 peculiar organic pigment. This pigment is not deposited in grains, but 

 is completely hyaline, and imbibes the whole tissue of the retina at this 

 spot, with the exception, according to Schultze, of the two external 

 layers, which remain colorless. 



At its centre is a minute depression, the fovea centralis, where, 

 owing to its steeply sloping sides, the retina is reduced, at the bottom 

 of the fovea, to less than one-half its usual thickness. The macula 

 lutea becomes perceptible, in ophthalmoscopic examination of the eye 

 with a moderate illumination, as a yellowish spot, less brilliant than the 

 rest, in w T hich the position of the fovea centralis is marked by a peculiar 

 colorless reflection. The macula lutea, and especially the fovea cen- 

 tralis, is the point of most distinct vision, where the image of an object, 

 fixed by the eye in the direct line of sight, falls upon the retina. It is 

 well known that external objects are seen with perfect distinctness only 

 when their images fall in the immediate neighborhood of the optical 

 axis at the fundus of the eyeball. Outside this region, the perception 

 of their figure is more or less imperfect. According to the observations 

 of Donders, confirmed by Helmholtz, if, while the retina is illuminated 

 by the ophthalmoscope, the person under observation fixes the eye in 

 succession upon several different objects, or upon different points of the 

 same object, the minute reflection which marks the fovea centralis 

 always places itself upon the part of the optical image fixed by the eye ; 

 and this appearance is so constant that the observer can tell with cer- 

 tainty, from the place occupied by the reflection, what point of the 

 object has been fixed in the direct line of sight. 



The evident importance of the macula lutea and the fovea centralis, 

 in the exercise of vision, gives a special interest to the anatomical 

 structure of this part of the retina ; and the researches of microscopic 

 anatomists have shown that its structure presents peculiarities fully 

 corresponding with its physiological endowments. 



The macula lutea is distinguished, in the first place, by the fact that 

 the superficial layer of optic nerve fibres is absent. Those fibres, ac- 



