254 SENSE OF SIGHT. 



of light from an object can never fall, at the same time, upon the in- 

 sensible point of each eye. The defect in vision is, consequently, never 

 experienced except in such experiments as those performed by Mariotte. 

 In one of these he succeeded in directing the rays to the insensible point 

 of both eyes at once. He put two round papers at the height of the 

 eye, and at the distance of three feet from each other. By then placing 

 himself opposite them, at the distance of twelve or thirteen feet, and 

 holding his thumb before his eyes, at the distance of about eight inches, 

 so that it concealed from the right eye the paper on the left hand, and 

 from the left eye the paper on the right, he looked at his thumb steadily 

 with both eyes, and both the papers were lost sight of. These experi- 

 ments show, that there is a part of the retina or optic nerve, which is, 

 in each eye, insensible to light; and that this point punctum csecum 

 is on the nasal side of the axis. No sooner, however, had Mariotte 

 published an account of his experiments, than it was decided that this 

 spot was the basis of the optic nerve ; a conclusion was accordingly 

 drawn, that the nerve is incapable of distinct vision, and this conclusion 

 has been embraced, without examination, in many of the books on optics 

 to the present time. Although probable, however, it is by no means 

 certain that the light, in these cases, falls upon the base of the nerve. 

 The direction in which the ray proceeds is such that it is reasonable to 

 suppose it does impinge there: the suggestion of M. Tillaye, 1 that it 

 falls upon the yellow spot of Sommering, can only be explained by 

 presuming him to have been in utter ignorance of the situation of the 

 yellow spot, which, we have seen, is on the outer side of the nerve. 



But, granting that the light falls at the base of the optic nerve, it 

 by no means demonstrates, that the nerve is incapable of receiving the 

 impression. It has been already shown, that the central artery of the 

 retina penetrates the eye through the very middle of the nerve; and 

 that through the same opening, the central vein leaves the organ. It 

 is probable, therefore, that, in these experiments, the ray falls upon the 

 bloodvessels, and not upon the medullary matter of the nerve; and if 

 so, we could not expect that there should be sensation. That the in- 

 sensible spot is of small magnitude is proved by the fact, that if a candle 

 be substituted for the round paper or wafer, the candle does not disap- 

 pear, but becomes a cloudy mass of light. Daniel Bernouilli 2 it is 

 t ru e considered the part of the nerve insensible to distinct impressions 

 to occupy about the seventh part of the diameter of the eye, or about 

 the eighth of an inch; but there must have been some error in his cal- 

 culations, for the optic nerve itself can rarely equal this proportion. 

 The estimate of Le Cat, 3 who was himself a believer in the views of 

 Mariotte, that its size is about one-third, or one-fourth of a line, is 

 probably still wider from the truth in the opposite direction. Simple 

 experiment, with two wafers placed upon a door at the height of the 

 eye, shows clearly, that both the horizontal and vertical diameters of 

 the spot must be larger than this. 4 



' Adelon, Physiologie, 2de edit., i. 448, Paris, 1829. 



* Haller, Element. Physiolog., xvi. 4, 4. 



a Traite des Sens, p. 166, Paris, 1767; or English translation, Lond., 1750. 



* Medical and Physiological Problems, by William Griffin, M.D., and Daniel Griffin, M. D. 

 p. 113, Lond., 1845. 



