22 



THE FUNDUS OCULI OF BIRDS 



in food-searching in such a fashion that they 

 are enabled to see one object very distinctly 

 and neighboring objects (in a restricted field 

 of vision) more distinctly than is permitted 

 by the comparatively insensitive retina of 

 the remaining fundal areas. 



Infulapapillary angle. The relations, es- 

 pecially of fundal position, between the optic 

 nerve entrance, the pecten, and the areas of 

 distinct vision are, as we have already seen, 

 of some biological importance. Upon the 

 locality of the macular regions depend, in 

 great measure, the quality and precision of 

 the eyesight of each particular bird. That 

 one may facilitate references to the location 

 in the eyeground of the retinal band the writer 

 proposes to measure what he has termed the 

 infulapapillary angle (infula, a band, papilla, 

 the optic disc), made by the junction of the 

 major axis of the ovoid entrance of the optic 

 nerve prolonged until it meets the retinal 

 band on the nasal side of the point of contact. 

 In the case of the Hudsonian Curlew (see 

 Fig. 125) it is 50°. 



The Optic Nerve of Birds 



In all Vertebrates the optic nerve has the 

 form of a cord, sometimes very short, as in 

 Birds, and occasionally long, as in Man, 

 which extends from the optic foramen to the 

 posterior segment of the eyeball, where its 

 expansion forms the retina. 



In Birds, whose ocular movements displace 

 the eyeball very little, the nerve takes the 

 form of a short, almost rectilinear cord; while 

 in animals with mobile eyes, such as the large 

 Mammifers, the nerve is long, round and 

 S-shaped, to follow the displacements of the 

 posterior hemisphere without pulling or drag- 

 ging. Its diameter greatly varies; it may 

 reach 8 mm., as in the Whale. The rule is 

 that it roughly corresponds in size and length 

 to the volume of the eye-ball. 



As Kalt says, the pia mater forms the 

 neurilemma or sheath of the optic nerve. 

 The multiple partitions which it sends into 

 its substance, divide the nerve into bundles; 

 the dissepiments anastomose with each other. 

 This is the type well known among Mammals. 

 Birds exhibit one peculiarity; at the inner 



aspect of the nerve the pial partitions pene- 

 trate perpendicularly into the substance of 

 the nerve and do not anastomose until they 

 reach about half way into the thickness of 

 the cord. Analogous perpendicular parti- 

 tions, but very short, exist sometimes on the 

 external side. The result is that the nerve 

 has a foliated appearance, especially marked 

 on the internal side. 



The Pecten, Marsupium, Fan or Comb 

 of Birds 



By means of the ophthalmoscope and direct 

 illumination there is seen, below and exter- 

 nally, in the fundus of every bird, a black 

 mass of variable form. When the free edge 

 of this organ is illuminated one notices a 

 longitudinal strip with undulating edges. 

 If one throws the light upon the base of this 

 object — the pecten or comb — the papilla 

 looks like a sparkling white area whose mar- 

 gins are edged with black, the center being 

 occupied by the irregular, black mass. 



Fig. 13 



Diagram Showing the Relations of the Optic Nerve 

 Entrance to the Pecten and the Basilar Artery and 

 Vein in the Sparrow. (Wood and Slonaker.) A, 

 artery to pecten, which sends a branch along each fold; 

 ch, choroid; on, optic nerve; pect, pecten; ret, retina; 

 scl, sclerotic; v, vein from pecten, which receives a 

 branch from each angle of the folds. 



