24 



THE FUNDUS OCULI OF BIRDS 



Franz does not believe that the fine struc- 

 ture of the bird's pecten has, as yet, been fully 

 described. The structural arrangement in 

 Struthio shows it to be different from other 

 birds, where it springs as a sort of wrinkled 

 leaf from the papilla and has an enlargement, 

 which he terms the "bridge," which runs 

 along the entire margin of the pecten. See 

 Figure 24 and Plate I. The folds are 

 welded together by this bridge, so that they 

 can only be smoothed out by cutting away 

 the latter. 



The histological structure of the Bird's pecten 

 has been investigated by Mihalkowics, Denis- 

 senko, Bernd and others. Bernd claims that 

 the pecten is not primarily of mesodermal 

 origin and an appendix of the choroid, as was 

 formerly held (by Kessler, for instance), but 

 is really a derivative of the retina or of the 

 optic nerve; in short, an offshoot of the 

 central nervous system and, consequently, 

 of the ectoderm. 



Franz maintains that the pecten is a sense 

 organ, assisting in the regulation of the intra- 

 ocular pressure and the act of accommoda- 

 tion. On the upper surface of the pecten, 

 that is on the "bridge," Franz found fila- 

 ments that end there in club-shaped bodies, 

 structurally between pigmented nuclei and 

 cell nuclei, and he believes we may look upon 

 the pecten primarily as a glia-structure, an 

 opinion also held by Blockmann and v. 

 Husen. Still, Franz will not say, nor does 

 Blockmann, that there is to be found in the 

 pecten any convincing evidence of the pres- 

 ence of true nerve filaments. 



Franz and Hushke point out that the pecten 

 is all the more developed the more the bird 

 makes use of monocular, and less of binocular 

 vision; that the pecten of Song-birds is large 

 and rich in wave-like folds, while the much 

 faster flying Swallow, with better accom- 

 modation, has a more poorly developed pec- 

 ten. The Owls, also, with their small pecten, 

 have binocular fixation. 



Both in Birds and Reptiles there is a close 

 connection between the pecten and the vitre- 

 ous; in other words between that organ and 

 the margins of the hyaloid canal, or slit, in 

 which it is, as it were, inserted. 



Franz does not feel certain about the vas- 

 cular supply of the pecten of the Bird. He 

 believes, however, that the bloodvessel that 

 runs along the base of the pecten gives off 

 branches to the folds (as shown in Fig. 13) 

 and, to a less extent, to the pectinate bridge. 

 The main vessel is to be regarded as the 

 artery (afferent vessel) of the pecten. 



We find glia-growths like those of the avian 

 pecten emanating from the papilla in many 

 Mammals and which present the same char- 

 acters as these organs in Reptiles. In the 

 eye of Amphibians and Fishes there are no 

 such glia-growths. 



After many dissections and much considera- 

 tion of the finer anatomy of the organ Slonaker 

 and the writer do not regard the avian pecten 

 as anything more than a series of bloodvessels 

 with a deeply pigmented framework of glia 

 fibres and cells. So far we have been unable 

 to find in numerous serial sections of the 

 Sparrow's pecten (or in that of any other bird 

 so far examined by us) definite ciliated struc- 

 tures, sensory buds, or any other appearance 

 that would justify a belief other than that 

 this organ, deeply pigmented to absorb the 

 light rays, has any function apart from that 

 of a carrier of nutritive pabulum, chiefly to 

 the vitreous and anterior layers of the retina. 

 It may change its position, actual or relative, 

 during accommodation, but it has no other 

 connection with the accommodative act. 

 The covering of all its fan-like folds is con- 

 tinuous and identical with the hyaloid mem- 

 brane or membrana limitans of the retina. 

 We did not find in the Sparrow any anasto- 

 moses between the vessels of the pecten and 

 the other ocular structures. The branch of 

 the ophthalmic artery that enters the eye- 

 ball close to the optic nerve sheath (to pene- 

 trate the latter at the plane of the sclerotic 

 coat) is the only afferent vessel-supply of the 

 pecten that we were able to demonstrate, 

 either by gross dissection or in serial micro- 

 scopic sections of this region. 



In the same way there appears to be only 

 one efferent vein which may, however, have 

 received other venules before its exit from the 

 globe, several millimeters distant. These two 

 latter observations are, however, based on an 



