THE SENSES. 531 



the outer angle of the eye, and about 45 degrees in front of the plane 

 of the iris. On moving the candle alternately up and down, the field 

 of vision becomes filled with an abundant tracery of arborescent figures, 

 the counterpart of the retinal blood-vessels. The form of the vessels 

 is marked in purple-black, on a finely granular grayish-red ground. 

 The point of entrance of the vascular trunks may be seen, with their 

 two principal branches passing respectively upward and downward, 

 and breaking into ramifications of various curvilinear form. If the 

 candle be held motionless, the figures rapidly fade, since they are 

 only visible from the contrasts made by the shadows falling in 

 succession on different parts of the retina. 



As the blood-vessels of the retina are situated nearly at its anterior 

 surface, the motion of their shadows, perceptible on varying the posi- 

 tion of the light, gives a means of ascertaining how far behind this 

 surface the sensitive elements are situated. According to Mu'ller,* this 

 distance must be, in various cases, from 0.17 to 0.36 millimetre; and 

 the same observer finds the posterior layers of the retina distant from 

 its anterior surface from 0.20 to 0.30 millimetre. It is, therefore, one 

 or both of the posterior layers, namely, the rods and cones, and the 

 nuclei immediately beneath, in which luminous rays produce their 

 effect. 



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

 yellow spot of the retina, is an oval space, about 2 millimetres in trans- 

 diameter, V)ft\veen 2 and 2.5 millimetres outside 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 its temporal 

 side. It is distinguished from the remainder of the retina by its yellow 

 tinge, due to the presence of an organic pigment. 



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

 to its steeply sloping sides, the thickness of the retina is reduced, at its 

 deepest part, to less than one-half. Its position in the macula lutea, in 

 ophthalinoscopic examinations, is marked by a peculiar colorless reflec- 

 tion. The macula lutea, and especially the fovea centralis, is the point 

 of most distinct vision, where the image of an object, in the direct line 

 of sight, falls upon the retina. According to the observations of Bon- 

 ders, confirmed by Helmholtz, if, while the retina is illuminated by the 

 ophthalmoscope, the person under observation fixes the eye upon sev- 

 eral different objects in succession, the minute reflection which marks 

 the fovea centralis always places itself upon the optical image of the 

 object fixed by the eye ; and this appearance is so constant that the 

 observer can tell with certainty, from the place occupied by the reflec- 

 tion, at what object the sight is directed. 



The importance of the macula lutea and fovea centralis, in the exer- 

 cise of vision, gives a special interest to the anatomy of this part of 

 the retina; and microscopic researches have shown that it presents 



*In Helmholtz, Optique Physiologique. Paris, 1867, p. 289. 



