BIRDS 



415 



by an artery derived from the hyaloid system emerging from the optic 

 disc entirely separate from the choroidal circulation ; this artery runs 

 along the base of the pecten and gives off ascending branches to each 

 of the folds, whence the blood is gathered by large veins which combine 

 to pierce the sclera and the cartilaginous cup at about the level of the 

 middle of the pecten (Fig. 522). The walls of the capillaries contain 

 no muscle or nerve fibres and between 

 them lie epithelial pigment -containing 

 cells ; the consensus of opinion is that 

 there are no structures resembling sensory 

 end -organs as was suggested by Franz 

 (1908). 



The function of the pecten has excited 

 speculation ever since it was discovered ; 

 this has. indeed, been one of the great 

 puzzles in comparative ophthalmology 

 and, based on the dramatic differences in 

 its size and complexity in various species, 

 more than thirty separate theories as to 

 its possible use have been advanced. Un- 

 fortunately few of them are based on 

 physiological experiment. It is to be 

 remembered that the presence of the 

 structures described by Franz (19U8) — 

 cilium-like hairs along the free edge of the 

 bridge associated with bulbous cells with 

 nerve fibrils running between the pecten 

 and the nerve -fibre layer of the retina — 

 has never been substantiated ; there is no 

 evidence that the pecten is anythmg more 

 than a complex capillary network or that 

 it can be interpreted in any respect as a 

 sense organ. Whatever accessory func- 

 tions (if any) it may have, all authorities 

 are agreed that its main role is to assist in the nutrition of the retina 

 and the inner eye generally ; it is thus strictly comparable to the falci- 

 form process of teleostean fishes or the conus of lizards. The metabolism 

 of birds runs at a high rate ; their normal temperature, for example, 

 may be 2' to 14^ F above that of Mammals. The metabolism of the 

 cone-rich retina must be similarly high and, as we have seen, the size 

 and the complexity of the pecten vary closely with the diurnal activity 

 of the species concerned. Its nutritive function was proved by Abels- 

 dorff and Wessely (1909) who showed the high permeability of the rich 

 capillary system to the solutes of the blood, while its complex shape 



ON 



Fig. r)22. — The Structure of 

 THE Pecten. 



Sliowing its relations to the 

 entrance of the optic nerve and 

 its vasenhir connections. A, The 

 supplying artery which sends a 

 branch to each fold ; Ch, 

 choroid ; ON, optic nerve ; 

 P, pecten ; i?, retina ; »§, sclera ; 

 T^ efferent vein which receives a 

 branch from each angle of the 

 fold (Wood and Slonaker ; the 

 illustration is inverted). 



