16 



THE FUNDUS OCULI OF BIRDS 



The Avian Choroid 

 The choroid coat (choroidea) of Birds' eyes 

 is always very thin, even in large eyeballs, 

 but thicker than in the Mammal's eye. The 

 vascular, capillary meshes (choriocapillaris) 

 are of irregular size and rather narrow; in 

 Struthio the capillaries are 0.01-0.02 mm. 

 wide — broader than in other birds. The 

 spaces between the capillaries are, according 



is seen at its widest about the time of hatch- 

 ing (when differentiation takes place most 

 actively) and becomes thinner and remains 

 permanent when the bird is several months 

 old. 



A tapetum is absent in Birds, although the 

 Nubian Ostrich has a glass-like layer in the 

 choroid, of lamellated structure, capable of 

 reflecting light and producing color inter- 



Fig. 7 

 Macroscopical, Postmortem Diagram of the Infulabimacular Fundus and Orbit of the Tern (Sterna hirundo). 

 N, optic nerve entrance; P, pecten; Ft, fovea and area temporalis; Fn, fovea and area nasalis; Ab, band-like 



area. (After Slonaker.) 



to Franz, completely free of cells and pigment. 

 The amount of pigment in the external layer 

 is subject to variation. 



The thickness of this coat in Passer domesti- 

 cus is about 200 microns. The vessels of 

 the choriocapillaris measure from 10 to 20 

 microns in width; between them is a finely 

 punctate substance. This layer is 7 to 8 

 microns thick. Externally there is an endo- 

 thelial layer, then a net work of fine elastic 

 fibres without cells or pigment; finally, a 

 dense plexus of connecting cells. 



The nerves run within the suprachoroidal 

 space and send only fine filaments into trabec- 

 ular that form a cavernous system throughout 

 the whole coat. 



Slonaker found the choroid to be enlarged 

 and most vascular (see Fig. 1, just opposite 

 the fovea, especially in embryonal life and 

 in newly-hatched individuals. It is in the 

 former instance one of the indications of the 

 spot where the fovea will eventually form; 

 then it widens during the foveal formation, 



ference effects. This arrangement, however, 

 is only a retino-choroidal variation, and not a 

 true tapetum. 



The Retina of Birds 



The acuteness of the eyesight in Birds is 

 largely due to the fineness and close arrange- 

 ment of the retinal elements. See Fig. 9. 



Gustav Fritsch (Archiv f. Mikroskop. Anat- 

 omie, Vol. 78, p. 245, 1911) believes that true 

 rods and cones are found in Birds, just as 

 they are in Mammals, and they should be 

 designated as such. In the region of the 

 fovea centralis there is also a dwarfed, cone- 

 like form. The colored, retinal droplets of 

 Birds appear mostly in the rod elements, 

 although they are sometimes seen in conjunc- 

 tion with cones. Among the central cones is 

 also found, quite commonly, a body which is 

 colored brown by osmic acid. 



According to Franz (Das Vogelauge, 1911) 

 the layers of the avian retina are more sharply 

 defined than in any other vertebrate; even 



