214 INTRODUCTION TO GENERAL PHYSIOLOGY 



cornea, so as just to avoid touching it. Drop 0.9 per cent, saline 

 into the gap between the surfaces. The image will disappear, because 

 the curved refracting surface is no longer present. Remove the 

 glass slide, the image reappears. 



Structure of the Retina. The posterior half of the eye of a 

 rabbit, after removal of the vitreous humor, is fixed, and sections 

 cut of a strip of the retina and sclerotic together. 



Place a piece of retina in I per cent, osmic acid for a few hours, 

 and then in dilute glycerine for a day or two. Take a small 

 fragment and tease it with needles in dilute glycerine. Further 

 separation of the constituents is brought about by tapping the 

 cover-glass. Many of the points in the structure of the retina can 

 be made out by aid of the figures in the text-books of histology. 



Receptors for Position 



The small transparent fresh-water mollusc, Cyclas, has a statocyst 

 attached to the ganglion in the foot, and can be examined in the 

 living state under a low power of the microscope. 



Expose the cartilaginous skull of a skate or dog-fish. 



Notice the position of the semi-circular canals, one set on each 

 side, after removal of the roof of the skull and the brain. They are 

 contained within a mass of cartilage behind the eyes. Carefully 

 slice away the cartilage until the three canals become easily visible. 

 Note their position in the three dimensions of space. 



