80 Eye Colour in Birds 



produced by a differential colouring of the iris. In Lawes' Bird an inner 

 ring of deep iridescent blue is surrounded by an outer ring of yellow 

 pigment. This colour effect is brought about in two ways. If a vertical 

 section of the iris taken at right angles to the surface be examined under 

 a low power the iris in the outer or yellow zone will be found to be four 

 or five times thicker than in the inner or blue zone. This increased 

 depth of tissue in the peripheral zone, containing as it does the muscular 

 fibres and capillaries, partly accounts for the opacity of the structures and 

 prevents the posterior pigment from shining through, hence the absence 

 of a blue effect. In addition the anterior surface of the iris in this zone 

 is covered with a layer of yellow pigment cells. 



The inner or blue zone on the other hand is a mere thin membrane, 

 sharply defined from the outer zone by a step down as it were in thickness 

 on the anterior surface of the iris. The thinness and translucency of 

 the tissues in this inner zone readily allow the black uveal pigment to 

 shine through. The greater part of the blue effect is no doubt due to the 

 presence of this underlying black pigment seen through translucent 

 tissues. This however is not the whole explanation. A large portion of 

 this thin inner zone consists of very finely fibrillated spindle cells of 

 connective tissue type. The ends of these cells break up in hardened 

 specimens into fibrillae which separate into wisp-like fringes. These , 

 elongated cells lie over the muscle fibre cells, and if the anterior surface 

 of the iris in this situation be examined under a low power the parallel 

 fibrillae bring about a diffraction grating effect. This accounts for some 

 of the blue reflection of incident light. This can be shown by varying 

 the angle at which the light is allowed to strike the surface of the iris. 

 At certain angles the blue effect is produced quite independently of the 

 posterior black pigment and can indeed be obtained after this has been 

 removed. Thus, in the case of Lawes' Bird of Paradise a brilliant colour 

 effect is produced by a combination of three factors : (1) Thinness and 

 translucency of iris tissue allowing the uveal pigment to shine through. 



(2) The absence of anterior yellow pigment cells in the inner zone. 



(3) A peculiar physical conformation of the connective tissue cells in 

 this area. A parallel fibrillation of cells. acts as a diffusion grating and 

 causes light to be reflected from the anterior surface of the iris at a 

 certain angle as blue in colour. It would be interesting to extend this 

 enquiry to other species of Birds of Paradise and to individual birds of 

 both sexes at different ages. 



