C. J. Bond 73: 



Orpington are examples, the muscle-fibre cells themselves also contain 

 the yellow pigment granules. Bearing on this point, namely the presence 

 of pigment granules in striated muscle cells, it is interesting to note that 

 in some orange eyed birds the striated cells of the ciliary muscle also 

 contain yellow pigment, and this gives in some cases a distinct yellow 

 colour to the muscle which is quite recognisable to the naked eye. When 

 we recall the fact that the ciliary muscle has the same embryological 

 origin, and remains throughout individual development in close structural 

 and functional continuity with the muscles of the iris which act on the 

 pupil, it is a matter of interest to find that both share in the pigmentary 

 changes .concerned in the production of eye colour in some birds. It is 

 clear that the occurrence of pigment in the voluntary muscle fibres 

 entirely under cover of the sclerotic and unexposed to the action of light 

 cannot have any influence in sexual selection. It must be regarded like 

 many other pigmentary phenomena as due to an overflow of normal 

 metabolic activity into cells which under ordinary conditions are free 

 fi-om such changes. The deposition of pigment granules in the iris 

 muscle and in the ciliary muscle is chiefly found in domesticated birds. 

 I have not, so far, come across pigmentation of the ciliary muscle in any 

 wild bird. Such pigmentation is in fact a metabolic abnormality since 

 it occurs in cells which in wild bii*ds are normally free from pigmentary 

 deposition. The effect of the deposition of a mass of pigment granules 

 on the functional activity of a striated muscle cell is also a point of 

 interest. It would be interesting to know whether the change affects 

 only the older cells, it does not at any rate affect all the muscle cells even 

 in a heavily pigmented iris. Bearing in mind that the yellow iris of 

 the domesticated fowl is produced by the interaction of several factors : 

 (1) the presence of yellow pigment granules in connective tissue cells, 

 and (2) the deposition of yellow pigment granules in striated muscle 

 cells ; it becomes a point of interest to ascertain whether the yellow iris^ 

 in wild birds owes its colour to the same or different factors. The owls 

 afford perhaps the best examples of the purely yellow iris among wild 

 birds. 



In the Eagle Owl {Bubo bubo) the peculiar velvety bright yellow 

 appearance of the iris is produced by a heavy coating of the front face 

 of the iris with numbers of roundish or oval cells crowded with bright 

 yellow spherical pigment granules. These cells are present also in Scops 

 Owl {Otus scops). Probably in most, if not all of the yellow eyed owls, 

 the yellow colour is due to the presence of these cells. The cells them- 

 selves are fairly constant in size and outline. They are free from 



5—5 



