THE PECTEN AND ITS ANALOGUES 651 



and size, have made it seem unlikely, to most, that nutrition is its chief 

 purpose. So, a great deal of thought has been spent upon its interpre- 

 tation. One of the most recent and interesting theories has been dis- 

 cussed on pp. 365-7. It has been variously held to cast a shadow on the 

 retina, or not to do so; and the supposed shadow has been involved by 

 one investigator in movement-perception, and by others in the pre- 

 vention of monocular diplopia during binocular vision, or in the sup- 

 pression of the binocular field during monocular fixation. It has even 

 been considered to serve as a 'dark mirror', transforming a too-bright 

 image (cast upon it by the lens) into a comfortably-bright one (relayed 

 from it to the retina), and making it possible for a ground-feeding bird 

 to see an approaching hawk in the sky without looking upward. The 

 pecten has been believed to adjust intra-ocular pressure (by swelling and 

 shrinking) during accommodation or during changes in the altitude of 

 flight, to serve as a proprioceptive sense-organ for the regulation of 

 accommodation, or even to assist mechanically or hydraulically in the 

 deformation or displacement of the lens. It has been held to be primarily 

 a heat-radiator, of especially great value to arctic, alpine, and high-flying 

 birds. To it has been ascribed a function similar to that of the holostean- 

 teleostean chorioid 'gland' — the smoothing out of intra-ocular blood- 

 pulsations, analogous to the action of an air-chamber on a reciprocating 

 pump. 



To each of these theories so many objections stand in the literature 

 that we shall not consider them in detail here. Suffice it to say that it is 

 unlikely that the pecten casts a shadow outside of its own base — or casts 

 one where it would do any good; that its shape, volume, and position 

 have not been found to alter during accommodation; that its size does 

 not correlate with the coldness of the air to which its owner exposes 

 itself; that it could not conceivably reflect an image even as good as 

 those one sees in fun-house mirrors; and that no sensory netve fibers or 

 endings have ever been demonstrated in it. We can cling, however, to 

 the demonstrations by Abelsdorff and Wessely of a ready diffusibility 

 of blood solutes through the walls of its vessels (despite their peculiar 

 and thick hyaline coats) into the vitreous, and of its capacity for com- 

 pensatory hypertrophy following the surgical excision of the ciliary 

 processes. 



It is because the writer does not believe that the pecten has any 

 'ulterior' function — particularly, any function with a directive connec- 

 tion with the relationship of the eye to the environment— that the struc- 



