EYE OF THE ENGLISH SPARROW 383 



short, thick stump is Very different from the optic nerve in man. 

 The optic tracts lead directly from the chiasma to the large 

 optic lobes {op 2). There is a complete decussation of the optic 

 fibers at the chiasma. The fibers cross not as a network of 

 individual fibers, but in flat ribbon-like bundles which alternate 

 with each other. There are eight or nine of these bundles from 

 each side. This has been pre\'iously described by Meckel 



Fig. 19 Posterior view of the eyes of the sparrow after the brain, the skull 

 and lower mandible have been removed, to show the relations of the different 

 parts. The superior rectus of the right eye has been removed to show the nerves 

 and muscles lying beneath. C, chiasma; h, branch from ophthalmic to third 

 nerve; G, Gasserian ganglion; IM, inferior maxillary; 10, inferior orbital; L, 

 lacrimal nerve; LG, lacrimal gland; N, nasal nerve; 0, superior oblique; Op, 

 ophthalmic nerve; Q, quadratus; RE, rectus externus; RS, rectus superior; SM, 

 superior maxillary; SO, superior orbital; 3, 4, 5, and 6, the third, fourth, fifth 

 and sixth cranial nerves. X 5. 



(1816), Biesiadecki ('61), and Michel ('73) as characteristic of 

 birds. They further claim that the number of these leaf-like 

 bundles varies with different species of birds. Meckel found 

 from fourteen to fifteen such bundles from either side in the crow. 

 Figure 19 is a posterior view of the eyes and the nerves. The 

 chiasma is cut across just anterior to the hypophysis and shows 

 the ribbon-like bundles of fibers. Figure 20 shows the appear- 

 ance of the eye structures as seen in a vertical section through 



