THE PERCEPTION OF FORM 643 



the temporal fovea and its long axis running parallel to the palpebral 

 fissure so that the refraction through this axis is extremely myopic ; 

 this system is brought into play for aquatic vision when the bird dives 

 under water for its prey (Kolmer, 1924).^ 



(c) An extraneous alteration of the 02)tical system by the inter- 

 position of the nictitating membrane. This is a curious and unique 

 mechanism seen in diving ducks, loons and auks whereby the third 

 eyelid is brought over the cornea when the bird is immersed ; the 

 nictitating membrane has a transparent window with a high refrac- 

 tivity, so that when it is interposed in the visual axis the already 

 powerful intra-ocular accommodative mechanism is augmented. 



{d) A duplicated retina is a rare accommodative expedient seen in 

 the tubular eyes of some deep-sea fishes in which the relatively enormous 

 size of the spherical lens precludes any effective accommodative 

 adjustment ^ : the princij^al retina in the axial position is myopic 

 compared with the accessory retina situated close to the side of the 

 lens (Fig. 767). In addition to this static mechanism we shall see 

 presently that there is a supplementary dynamic component mediated 

 by a muscle of accommodation.^ 



(e) A sloping ramp-retina is a somewhat similar device whereby 

 the axial length of the globe changes continuously in the vertical 

 direction, being progressively further away from the lens in its superior 

 segment. Such a configuration is seen in some Selachians (Raja) (Fig. 

 768) and particularly in Ungulates (Franz, 1934). In the horse, for 

 example, which has no djTiamic accommodation, the axial retina which 

 is used for forward regard is emmetropic while the upper portion of 

 the retina, used for the near vision required in grazing, is myopic 

 (Fig. 769). A somewhat similar arrangement is seen in the ocelli of 

 some Invertebrates.^ 



(/) The corrugated retina of the larger bats (Megachiroptera) ^ 

 results in a considerable variation in the distance of the receptor 

 elements from the optical centre depending on whether they are 

 situated on the crests or the sides of a choroidal papilla or in the valleys 

 between them ; from the optical point of view this must ensure that the 

 images of objects situated at varymg distances will be focused on some 

 visual cells (Fig. 770). 



{g) An unusual length of the receptor elernents of the retina will have 

 the same optical effect for the image, while yet remaining within the 

 receptor layer, will be able to traverse a considerable axial distance 

 corresponding to a relatively great movement of an object in space. 

 An extreme length of the visual elements, as is seen in many deep-sea 

 Teleosteans or in nocturnal geckos (Fig. 433), is doubtless primarily an 



1 p. 65.5. 2 p. 323. 3 p. 646. 



« Fig. 106. ^ p. 459. 



