352 AMPHIBIA xil. 26- 



move the lens forward, whereas the muscles of the lens of teleostean 

 fishes move it backwards. Other fibres, the musculus tensor chorioi- 

 deae, run radially and around the lens. They may help the protractors, 

 and are probably the ancestors of the ciliary muscles of higher 

 forms. 



In amphibia living in the water the eye is based much more on the 

 fish plan and the lens is rounder. There are then no lids or lachrymal 

 glands and the eye is enabled to make an image, in spite of the absence 

 of the air-corneal interface, by a thickening of the inside of the cornea. 



Rods and cones are present in the retina, the former containing 

 visual purple, which may be red or greenish. The two sets of receptor 

 are apparently found throughout the retina in urodeles, but in Rana 

 there is a macular region in which the cones are in excess and this is 

 still further developed in Bufo. Study of the impulses in the optic 

 nerves of Rana shows that six types of detector operate upon the infor- 

 mation provided by the rods and cones. (1) Contrast detectors give a 

 sustained response when a sharp edge moves into the visual field ; (2) 

 convexity detectors respond to objects that are curved, the discharge 

 being greater the more curved (smaller) they are. These two types 

 together may be called 'on' fibres; (3) moving-edge detectors ('on/off' 

 fibres) respond with a frequency proportional to the velocity of move- 

 ment; (4) dimming detectors respond on reduction of illumination 

 ('off' fibres); (5) darkness detectors fire with frequency inversely pro- 

 portional to illumination. These types of fibre project to different 

 depths in the tectum as sheets of endings, and the arrangement of the 

 retina is accurately reproduced there although the fibres are interwoven 

 in the nerve (perhaps to prevent 'cross-talk'). Moreover, if the nerve 

 is severed the fibres regenerate in such a way as to reconstitute the 

 map. The sixth type of fibre is sensitive to blue light and is connected 

 with the thalamus. 



These operations serve to provide reports of the types of change 

 relevant to the animal. Thus the second type might be called 'insect 

 detectors', responding when a small dark object enters the field 

 and moves about intermittently. More complex visual discrimina- 

 tions are also possible, for example toads can distinguish between 

 shapes. 



The skin is probably sensitive to light in all amphibians : frogs react 

 to light even after removal of the eyes and cerebral hemispheres. This 

 skin sense is especially developed in certain cave-living urodeles, 

 Proteus, in which the eyes are not functional. A similar degeneration 

 also occurs in Apoda. 



