152 STEUCTURE AND CLASSIFICATION OF BIRDS 



suggest the inference that the brain of the cretaceous bird 

 furnishes an example of degeneration rather than of the 

 retention of an archaic character. 



The hemispheres, although smooth in the majority of 

 birds, show faint indications of what may correspond to 

 furrows in some others. In the duck, for example, as 

 figured by GrADOW (after Bumm), there is a central raised area 

 marked off by a furrow from the STxrrounding parts of the cere- 

 brum ; faint traces of the same occur in Buteo vulgaris. The 

 weight of the brain as compared with that of the whole body 

 has been studied by several observers, and according to their 

 results the Passeres and parrots take the highest place. 

 But Gadow justly remarks that ' the attempts to sort birds 

 according to the proportion of brain to body have led to no 

 practical results, chiefly because the variable conditions of 

 fat and lean subjects have not been considered.' 



The brachial and lumbar plexuses, particularly the 

 former, have been studied in a large number of birds ; 

 Ftj:^bringbr has published a quantity of drawings of the 

 former, but no classificatory results of reliability appear to 

 follow from the facts collected with so much diligence. The 

 brachial plexus varies in position and in complexity. The 

 former variations are largely correlated with the varying 

 length of the neck ; thus in Columba the first spinal nerve 

 entering into the plexus is the tenth, in Phmnicopterus the 

 seventeenth. The number of nerves which together form 

 the plexus varies from only three in Bucorvus to six in 

 Columba. 



The Eye. — The eye of birds presents many resemblances 

 to that of reptiles. The minute structure of the retina 

 presents many points of similarity, as also the ring of ossifi- 

 cations in the cornea, and the pecten. The latter is a folded 

 process of pigmented tissue which projects into the vitreous 

 humour through the choroidal fissure, which is in the embryo 

 the gap left between the edge of the optic cup and the lens 

 on one side. The pecten offers differences in various birds 

 upon which perhaps some little stress can be laid. In 

 Apteryx it is entirely absent. There are very few folds in 



