590 STUDIES IN ADVANCED PHYSIOLOGY. 



small opaque bodies in the vitreous humor throw against 

 the retina, which shadows we by mistake project outside of 

 the eye. These opaque bodies in the vitreous humor are 

 remnants of embryonic blood-vessels. 



(b) The figures of Piirkinje. These are figures pro- 

 duced by the shadows which the blood-vessels of the retina 

 throw against the retina. The blood-vessels lie in the outer 

 coats of the retina, and so whenever light enters the eye 

 they cast a shadow on the deeper sensitive layers. Ordi- 

 narily we do not notice this shadow because we have been 

 accustomed to it, as the shadows always fall on the same 

 points of the retina, the light entering always at the pupil. 

 In the same way we might become insensible to an object 

 resting on a certain portion of the skin if it should remain 

 there for any great length of time. When, however, the 

 object is suddenly placed on a new bit of skin it produces a 

 distinct sensation, so when the shadows of these blood- 

 vessels fall on a new part of the retina we at once distinctly 

 see them, and as usual project these shadows out into space. 

 If an individual go into a room lighted only with a lamp and 

 then move the lamp at one side of his head in such a way 

 that the light of the lamp pass through the white portion of 

 the eye on that side instead of through the pupil, he will 

 suddenly see, especially if he look against a white wall, a 

 branched system of vessels, and if the experiment is very 

 successful may even note the translation of the individual 

 corpuscles through these blood-vessels. The explanation 

 is apparent. The light entering through the sclerotic por- 

 tion of the eye causes the blood-vessels to cast a shadow in 

 a different direction than when the light normally enters 

 the pupil, and this shadow falling upon a new portion pro- 

 duces a sensation. These Purkinje figures may be pro- 

 duced upon suddenly emerging into the light after having 

 been confined for some time in a dark space, or after a 

 night's sleep upon suddenly opening the eyes. In this case 

 the retina where the shadow normally falls has been resting 



