324 THE BIOLOGY OF BIRDS 



Various functions have been ascribed to this pecten. 

 It has been regarded as a Hght fiher, protecting the retina 

 from too intense illumination. It has been credited with 

 regulating the transference of fluid from the anterior 

 chamber to the posterior chamber of the eye when there 

 is focussing for near vision, and it is certainly well-deveioped 

 in high-flying birds, birds of rapid flight, and birds of prey 

 where the accommodation of the eye has to take place very 

 rapidly. And it is small in nocturnal birds. Against this 

 view is the fact that the pecten occurs in some reptiles. 



Dr. Lucas notes that a strongly developed pecten creates 

 a large blind area in the eye, for it is heavily laden with 

 dark pigment and the rays of light that fall on it are naturally 

 absorbed. " This explains to some extent the constant 

 shifting of the head when the bird is on the watch, as the 

 visual field is considerably limited, the portion obstructed 

 being toward the upper outer field of vision." 



A very careful study of the pecten has been made by 

 Ebba von Husen (19 13), who comes to the conclusion that 

 it is mainly concerned in the making of the vitreous humour. 

 The tissue of the pecten is a network of coalescent cells, 

 like the neuroglia cells which form the scaffolding for nervous 

 tissue. But the pecten is not nervous or sensory. It is 

 covered with epithelial cells with their bases turned outw^ards ; 

 there is fluid in the intercellular spaces of the network of 

 anastomosing cells ; there are also blood-vessels. 



In development the pecten starts, as Berndt has shown, 

 from a mesodermic keel and an ectodermic covering, both 

 of which contribute to the making of the embryonic vitreous 

 humour. In this way the pecten will aid in the nutrition 

 of the eye and will assist in regulating the tension. 



It must be pointed out that some investigators insist 

 that the pecten is a definitely nervous structure. Thus 

 V. Franz (1908) is most circumstantial in his declaration 

 that the pecten is not a derivative of the choroid, but of the 

 optic nerve. Apart from the blood-vessels which arise 

 from the ophthalmic artery, the pecten, according to Franz, 

 consists entirely of nervous tissue, and bears sensory pro- 



