626 TEXT-BOOK OF PHYSIOLOGY. 



are in<fflvyble to the impact of the ether vibrations, and require 

 for their excitation some intermediate form of energy. That this is 

 the case was shown by Bonders, who reflected a beam of light on 

 the optic nerve at its entrance without the individual experiencing any 

 sensation of light. This region, occupied only by the optic-nerve 

 fibers and devoid of any special retinal elements, is therefore an 

 insensitive or blind spot. The diameter of this spot is about 1.5 mm., 

 and occupies in the field of vision a space of about 6. It is situated 

 about 3.5 mm. to the nasal side of the visual axis. Its existence can 

 be demonstrated by the familiar experiment of Mariotte, which con- 

 sists in placing before the eye two objects having the relation to 

 each other as in Fig. 301. With the left eye closed and the right eye 

 directed to the cross, both objects may be visible. But by moving 

 the figure away from or toward the eye, there will be found a distance, 

 about 30 cm., when the circle will be invisible. This occurs when 

 the image falls on the optic nerve at its entrance. The experiment of 

 Purkinje as described in the following paragraph demonstrates also 



FIG. 301. DIAGRAM FOR OBSERVING THE SITUATION OF THE BLIND SPOT. 



(Helmholtz.) 



the fact that the sensitive portion of the retina is to be found only in 

 the layer of rods and cones. 



It is well known that the blood-vessels of the retina are situated 

 in its innermost layers a short distance behind the optic-nerve fibers. 

 Owing to this anatomic arrangement, a portion of the light coming 

 through the pupil will be intercepted by the vessels and a shadow 

 projected on the layer of rods and cones. Ordinarily, these shadows 

 are not perceived, for the reason that the shaded parts are more 

 sensitive, so that the small amount of light passing through the vessels 

 produces as strong an impression on this part as does the full amount 

 of light on the unshaded parts of the retina, and perhaps because the 

 mind has learned to disregard them. But if light be made to enter 

 the eye obliquely, the position of the shadows will be changed, when at 

 once they become apparent. This can be shown in the following way : 

 If in^ a darkened room a lighted candle be held several inches to 

 the side and to the front of the eye, and then moved up and down, 

 there will be perceived, apparently in the field of vision, an arbores- 

 cent figure corresponding to the retinal blood-vessels. This is due 



