18 



THE FUNDUS OCULI OF BIRDS 



The layer of ganglion cells. Cajal differ- 

 entiates the following types of ganglion 

 optical cells: (a) single-layered cells, spread- 

 ing out over the internal reticular layer, 

 partly multipolar, but mostly belonging to 

 the reticularis; (b) many -layered, multipolar 

 cells. The smallest are plentiful in the 

 Passeriformes; i.e., the Sparrow, Chaffinch, 

 Greenfinch, etc. 



It should be noticed that Dogiel expressed 

 the opinion that those cells which in form, 

 size and character of branching of the pro- 

 toplasmal and cylindrical processes, belong 

 to a certain characteristic type, anastomose 

 with each other, or form a "colony." 



The relatively thick, optic connective tissue 

 receives single, dissimilar, thick filaments, 

 which, originating centrifugally from the 

 optic nerve, intermingle with the internal 

 plexiform tissue and end, according to Cajal, 

 on the level of the amacrine cell-layer. 



Specific differences in retinal elements. As 

 in almost all of the sub-classes of vertebrates, 

 there are differences in the retinae of Birds, 

 especially in the tissues of the rods and cones. 



The numerical proportion of rods to cones 

 in different species is quite marked. Richest 

 in cones are the diurnal Birds of Prey. Cor- 

 responding to these in Night Owls are "rod- 

 retinae." M. Schultze thinks that while 

 Owls have many long rods, they have few 

 cones; Krause believes that the cones are 

 invisible on account of the number and length 

 of the rods, but that they are no less numer- 

 ous in the Owl than in the Falcon. 



Geese have, according to Krause, many 

 long rods and stand in that respect midway 

 between Owls and diurnal Birds of Prey. 

 The Flamingo has the longest of all rod ex- 

 tremities. The Heron has many thick rods. 



Krause seeks to weaken the force of the 

 statement of Schultze, that Nocturnal Birds 

 are distinguished by the preponderance of 

 cones and a diminished number of rods, by 

 pointing to the discovery of Heinemann that 

 in (the nocturnal) Nycticorax the rods are 

 scarce; that the Swallows, Hirundo rustica 

 and Chelidon urbica, in the relative increase 

 of their cones resemble the Owls, while 

 Athena noctua possesses even more cones than 



rods. According to Hess, the number of 

 cones in the Owl's retina reaches 1 to 2^ 

 millions. 



However the teaching of Schultze has much 

 in its favor; insofar as that the hypothesis 

 supported by it, viz., that cones are really 

 color-percipient organs (the rods perceiving 

 light and form) finds proof in the fact that 

 rods alone are found only in the retinae of 

 deep-sea Fishes and Whales. 



Regarding the possession of retinal oil 

 droplets, ellipsoids, paraboloids and hyper- 

 boloids, there are, according to Krause, many 

 differences in species. In Cardinalis virgin- 

 ianus and Fringilla spinus, he differentiated 

 two kinds of cones; in the Hen three outer 

 double cones. Fringilla spinus shows a blue 

 oil droplet in many of its double cones. 



The layer of nerve filaments receives in the 

 Pigeon and a few other birds partly medul- 

 lated fibres. Franz finds in many birds that 

 the layer of nerve filaments in the vicinity 

 of the optic foramen corresponds to what in 

 Mammals are called "spider cells." 



Pigment Movement in the Retina of Birds 

 Although this strange phenomenon probably 

 occurs in most vertebrates, and to a slight 

 extent in some invertebrates, yet it is in birds 

 that it is best studied. As Kalt remarks, the 

 retinal pigmented epithelium forms fringes 

 which, under the influence of light, descend 

 to the external limiting membrane. In the 

 dark they do not move past (at least in the 

 Pigeon) the middle of the rods and cones. 



The purple of the retina is missing, accord- 

 ing to Kiihne, from the rods and cones of the 

 Hen and Pigeon. It exists in large quantities, 

 however, in the external segments of the rods 

 of the Screech Owl, while the cones are devoid 

 of it. 



As Kiihne points out, the cones have vari- 

 able dimensions, and are single or double. 

 At the end of the internal segment they have 

 a single, oily, colored ball or droplet, which, 

 however, is not found in the interior of the 

 accessory cones. In a fresh retina the balls 

 show immediately with their various tints. 



As far as the color of the retinal oil-droplet 

 of Birds in general is concerned, Schultze 



