BIRDS 



411 



Perrault (1676) whose observation was elaborated by Petit (1735), is 

 a structure peculiar to Birds and forms the most dramatic feature of 

 the fundus when viewed oi3hthalmoscopically. It ajDpears in the ven- 

 tral part of the fundus as a black velvety mass rising from the elon- 

 gated optic disc, heavily pigmented particularly towards its apex. 

 Beautifully and elaborately convoluted, it projects freely into the 

 vitreous, usually moving undulatingly with movements of the gel (Fig. 

 506). Morphologically two main types occur : 



Figs. 507 and 508. — The Vaned Type of Pecten. 



Fig. 507. — Diagram of the pecteii of 

 the ostrich, Struthio ( X 5). 



Fig. 508. — Section jmrallel to tlie base 

 showing the central web and the 

 lateral vanes (after Franz). 



(1) The varied type. In Pal^eognathfe (except the cassowary and 

 the kiwi) the organ is composed of a central vertical panel with laterally 

 disposed vanes (Figs. 507-8). In the kiwi. Apteryx, it has a form resem- 

 bling the conus of lizards (Fig. 512). 



(2) The pleated type. In Neognathse (and the cassowary) the 

 whole organ is pleated upon itself like an accordion, the convolutions 

 being held in place by a band-shaped apical bridge running along the 

 top (absent in the owl-^) ; if this is cut away, the pleats can be 

 freely smoothed out (Fig. 509). 



Although always built on much the same general plan, the 

 pecten varies considerably in shape, size and the number of folds. 

 To a certain extent its size and complexity vary with the visual acuity 

 of the bird and its activity in daylight (Wagner, 1837 ; Virchow, 

 1901) ; active diurnal birds therefore tend to have a large and many- 

 folded organ, nocturnal varieties a small and simpler structure. 



The number of pleats varies between 5 and 30 (Wood, 1917 ; Kajikawa, 

 1923 ; Franz, 1934) (Figs. 510-11) ; 14 to 27 in the average ground -feeding or 



% 



The kiwi 

 Apteryx 



