l6o THE HUMAN BODY. 



plished in the retina alone. Other causes besides the contact 

 of luminous waves may excite the retina; thus the pressure of 

 the finger on the eye for example, and the disturbance re- 

 sulting from a fall or a blow on the head, the action of elec- 

 tricity, and certain affections of the eye and brain, give rise, 

 in the absence of natural or artificial light, to luminous 

 images varying in form and intensity. Light produced 

 under these conditions is called "retinal light." 



Like the optic and the other special nerves of the organs 

 of sense, the retina has a special sensibility; it receives the 

 impression from the light and transmits it to the brain, but it 

 is not itself sensitive to touch. No mechanical irritation 

 causes it the slightest pain. In a normal condition, the action 

 of a too brilliant light, and in certain affections of the eye 

 and brain, the least ray will cause a painful sensation, but 

 this pain must be referred either to the encephalon or to 

 the nerves of the iris or of the ciliary circle, independent of 

 the retina and the optic nerve. 



Punctum cctcum or blind point. Mariotte was the first to 

 recognize that all parts of the retina were not equally sensi- 

 tive. According to most authors, a limited portion of this 

 membrane, corresponding to the papilla of the optic nerve, 

 is totally insensible to light. M. Longet admits that there 

 it has, at any rate, a very obtuse sensibility. This "blind 

 point" is the only one on the internal surface of the eye 

 which is not covered with pigment. 



If we trace two figures on a horizontal line on a piece of 

 paper placed vertically (fig. 36, p. 161), and then shut the 

 right eye, and fix the left on the right figure, at certain dis- 

 tances we can see both of them more or less distinctly; but 

 varying the distance of the paper from the eye, there is a point 

 when we only see the figure upon which the eye is fixed ; the 

 other disappearing entirely, but it reappears when we change 

 the position of the paper, or cease to look fixedly at one figure. 

 The greater the distance between the two figures, the greater 

 must be the distance of the paper from the eye, in order to 

 render one of them invisible. The image of the invisible 

 one is then projected on the blind point, and it reappears, 

 when by the displacement of the paper the angle which its 



