BIRDS 



407 



There are only incidental differences between these muscles in the various 

 species of Birds. In diurnal predators they tend to amalgamate on the shortened 

 nasal side and sejtarate on the lengthened temporal side; in the swift, Micropiis, 

 the entire ring is symmetrical. In nocturnal predators Crampton's mviscle is 

 well -developed and Briicke's muscle is small and may be almost absent (most 

 owls, Strigidfe). Since deformation of the cornea is of no value in aquatic 

 vision, CramjDton's muscle is small in water-birds (as in diving ducks) or absent 

 (as in the cormorant, Phalacrocomx), while in compensation and to attain the 

 necessary accommodative range to change from aerial to aquatic vision, Briicke's 

 meridional muscle is massive in these types and may even be supplemented by 

 circular fibres as in the mviscle of Miiller in the human eye (cormorant ; gamiet, 

 Sula hassana) (Ischreyt, 1914). A muscle homologous to the transversalis 

 muscle of lizards has been described in the pigeon (Zalmann, 1921). 



The iris is remarkably thin at its cihary attachment where it is 

 reduced ahnost to the two ectodermal layers, tliickens towards its 

 mid -point and tliins again at the pupillary margin. The ectodermal 

 layers are both heavily pigmented and give rise to the striated sphincter 

 and dilatator muscles. These are extremely active and unusually 

 powerful, particularly the former which is richly vascularized ; it 

 braces the iris against the periphery of the lens thus assisting the 

 ciliary musculature in the moulding of this tissue in the act of 

 accommodation, at the same time confining the deformation to the 

 axial region. The sphincter is particularly well developed in some 

 amphibious birds (cormorant, Phalacrocorax ; shearwater, Puffinus ; 

 gannet, Sula ; and the sea-gulls, Laridse, etc.) ; in the cormorant, 

 for example, it is able to force the axial portion of the soft lens as a 

 conical protrusion through the pupillary aperture. The dilatator 

 fibres form a complete layer behind the sphincter, running into the 

 ciliary region, their unusually great development being perhaps due 

 to the probability that they also play a part in compressing the lens 

 on accommodation and provide a fixed anchorage for the sjjliincter 

 (Grynfeht, 1905 ; Hess, 1910 ; Zietzschmann, 1910 ; Wychgram, 

 1914 ; Zalmann. 1921 ; Welmer, 1923 ; Anelli, 1934). In colour the 

 iris is variegated. Most of the song-birds have a brown pigmentation 

 resembling the mammalian tyj)e ; but in other species brilliant 

 lipochrome pigments are common, particularly yellow, bright blue and 

 green, often giving the eye a bright colour-contrast with the rest of 

 the body (Balducci, 1905) (Plate XI, Figs. 1 to 5). 



This advertising habit is carried a stage further in the I'eriivian guano 

 coi-morant, PhalacrGCorax hougainvillii, the eye of which, with its dun-brown iris, 

 is surrounded by a ring of naked skin coloured bright green. The colour of the 

 iris is yellow in most owls, the jaigeon, Columba, and the starling, Lamprocolius 

 chalybeus ; bright blue in the nocturnal oil-bird, Steatornis ; sky-blue and 

 chocolate in the yellow hang-nest, Cacicus cela ; green in the cormorant and the 

 duck, Dendrocygyui, and the flamingo, Phoenicopterns ruber ; white peripherally 

 and chocolate with white concentric lines in the pupillary part in the budgerigar, 



The swift 

 Micropus 



The gaiiiiet 

 Sula 



The shearwater 

 Puffinus 



The flamingo 

 Phanicopterus 



