106 PRACTICAL PHYSIOLOGY 



CHAPTER XXVII. 

 THE RETINA. 



1. The Blood-vessels of the Eetina. The blood-vessels supplying 

 the retina are distributed to the anterior portion of the retina, the main 

 vessel entering the eyeball at the spot where the optic nerve 

 passes in. These blood-vessels then lie between the vitreous and the 

 sensitive part of the retina, and under certain circumstances they 

 may throw shadows upon this portion of the retina. 



EXPERIMENT I. Purkinje's Figures. Make the subject of the 

 experiment turn one eye inwards, and with a lens concentrate a good 

 light upon the exposed sclerotic, focussing the light so as to make a 

 small but strongly-illuminated area. Let the subject look towards a 

 dark wall. Give the lens a gentle rocking or circular movement. The 

 field will appear to the subject as reddish-yellow, and dark figures will 

 be seen by the subject appearing in the field, which branch and have 

 the character of the retinal blood-vessels, of which they are really the 

 shadows. In the direct line of vision a small area will be seen free 

 from these branching shadows. This is the yellow spot. 



EXPERIMENT II. Through a pinhole in a card held close to the eye, 

 look at a brightly and evenly-illuminated surface, as a white cloud or a 

 sheet of thin white paper held in front of a lamp. Give the card an 

 up-and-down movement, and a number of vessels will be seen running 

 horizontally in general. Move the card from side to side, and 

 vertically-running vessels will be apparent. Give the card a circular 

 movement and the general distribution will be visible. Note that in 

 the direct line of vision is a small area in which no vessels are seen> 

 the macula lutea or yellow spot. 



EXPERIMENT III. Remove the objective from a microscope, arrange 

 the mirror for a good light, and move the microscope in the same way 

 as the card was moved in Experiment II. The results will be as in that 

 experiment. 



In all these experiments the movement of the light or the illuminated 

 field is essential. The retina appreciates these shifting shadows better 

 than if they were continually applied to any fixed point of its surface. 

 Further, a moving object will arouse attention more readily than one of 

 constant position, which tends to be neglected. 



2. The Circulation in the Blood-vessels of the Eetina. EXPERI- 

 MENT. Look through a thick piece of blue glass at a white cloud. 

 Many finely-illuminated points will be seen traversing the field. These 



