410 



THE EYE IN EVOLUTION 



The goat-sucker 

 Caprimulrjus 



The bald eagle 

 Haliaetus 



The pelican 

 Pelecanus 



The zonular fibres arise over a wide area from and between the ciliary 

 processes (Teulieres and Beauvieux, 1931). 



Between the annvilar pad and the main body of the lens a small vesicle 

 filled with albuminovis fluid remains as a remnant of the embryonic lens vesicle 

 - — the CAVUM LENTicuLi of Franz (1934). To some extent this may be an artefact 

 of preparation, but it probably aids the process of deformation when the lens 

 is squeezed by the ciliary processes. 



Ophthalmoscopically, the fundus oculi of Birds presents a remark- 

 ably constant picture which has been extensively studied and 

 beautifully illustrated in a unique volume by Casey Wood (1917). The 

 background of the fundus is usually fairly uniform and almost invari- 

 ably besprinkled with pigmented dots of yellow or brown. Its colour 

 varies from grey or a slate-colour to orange and red. In general, the 

 fundi of diurnal birds are characterized by a grey or light brown 

 background (such as the bluebird, Sialia) (Plate XII, Fig. 3) ; that of 

 nocturnal birds tends to be yellow, orange or reddish (such as the kiwi, 

 Apteryx, the tawny owl, Strix aluco, the European night-jar or goat- 

 sucker, Caprimulgus europceus) (Plate XII, Figs. 1, 2, 4) ; a multi- 

 coloured background is more rare (buff and dull red in the American 

 ostrich, Rhea; dark reddish-brown and grey in the bald eagle, Haliaetus 

 leucocephalus). Frequently choroidal vessels may be seen shining 

 through, an appearance usually confined to a small segment of the 

 fundus in its ventral part, as in the Australian pelican, Pelecanus 

 conspicillatus, and the kestrel, Falco tinnunculus (Plate XII, Fig. 5) ; 

 more rarely the vessels are generalized, as occurs in the tawny owl, Strix 

 aluco (Plate XII. Fig. 2) ; as a rule these 

 vessels are most apparent in nocturnal 

 birds. Nerve fibres are usually not seen 

 ophthahnoscopically ; they are rarely 

 visible in nocturnal birds, but in divn-nal 

 types they often radiate outwards from the 

 disc, sometimes inconspicuously and run- 

 ning for a short distance only (Plate XII, 

 Fig. 4) but occasionally covering a wide 

 area (Plate XII, Fig. 3). The optic disc is 

 invariably white and elongated into a long 

 CAUDA (except in the kiwi, Apertyx) which 

 runs ventrally along the line of the foetal 

 fissure (v.Szily, 1922 ; Mann, 1924 ; Uyama, 

 1936) ; it is, however, almost entirely ob- 

 scured by the pecten. 



The PECTEN, 1 originall}^ described by 



1 The name is derived from the French peigne (a comb), but in view of the fact 

 ! there are no separate teeth in the structure, a more happily chosen name is the 

 • nan Fdcher (a fan). An early narne was Marsupium (see Crampton, 1813). 



Fig. .506. — Vertical Section 

 OF THE Right Eye of a 

 Goose. 



Showing the temporal half 

 of the globe. The jDecten 

 arising from the elongated optic 

 disc is seen (Thomson). 



