ANATOMY OF THE FUNDUS ORGANS IN BIRDS 



19 



differentiates mainly (a) colorless, (6) yellow 

 to reddish-yellow and (c) red droplets. 

 Again, he finds a difference between Day 

 Birds and Owls in that the red are lacking 

 in the latter while the pale yellow and color- 

 less varieties are more plentiful. 



According to Kalt, Waelchli distinguishes 

 four varieties of colored oil droplets in the 

 avian retina; red balls disseminated over the 

 whole membrane; orange, or yellow balls, 

 whose distribution is the same; greenish- 

 yellow ones seen at the periphery; and un- 

 colored ones spread in small quantities over 

 all. Osmic acid colors all these balls black. 

 The coloration is particularly intense in the 

 macular region. 



The locality in which the colored droplets 

 are found varies with each tint. The green 

 balls are situated nearest the external sur- 

 face of the retina, the red come next, then 

 the yellow, and finally the colorless balls. 



A chemical examination of the colored 

 globules in the cones of birds was made by 

 Kiihne. A hundred retinas of the Hen were 

 treated with alcohol and ether. The fatty 

 material thus dissolved (colored red) gave, 

 with soda, a soap from which was extracted 

 by petroleum ether a green coloring matter; 

 by sulphuric ether an orange material; by 

 turpentine a deep rose colorant; this last one 

 is, however, insoluble in sulphide of carbon. 



Kiihne gave the names of chlorophane, xanth- 

 ophane and rhodophane to these pigments. 



The Areas of the Retina in Birds. Avian 

 areae centrales are of greater variety — espe- 

 cially regions of distinct vision — and in 

 greater number than in other Vertebratse. 



As described (see Chapter VII, page 59) and 

 depicted elsewhere (Figs. 110 to 115 inclu- 

 sive) the writer believes that there are at 

 least six forms of these arese in Birds. 



According to Chievitz (1891) and Slonaker 

 (1897) there is at least one round macular 

 region with a fovea in practically all Birds. 

 This single area of monocular vision (see 

 Figs. 110 and 111) is in most Birds situated 

 somewhat centrally, slightly to the temporal 

 or the nasal side. 



In many species, diurnal Birds of Prey for 

 example, besides a central fovea in the nasal 



aspect of the retina, fovea nasalis, there is a 

 fovea temporalis, which with the fovea tempo- 

 ralis of the opposite eye may serve for binocu- 

 lar vision. 



The area centralis retina?, macula lutea or 

 macular region, is found in all classes of 

 Vertebrates and its presence is the rule, espe- 

 cially in Reptiles and Birds. 



The terms commonly applied to the various 

 parts of the macular region are as follows: 

 the whole area is designated the macula, or 

 area centralis or "yellow spot;" the more 

 sensitive, smaller area, generally disposed in 

 the geometrical center of the macula, is called 

 the fovea. Not infrequently there is seen a 

 still smaller point at the bottom of the foveal 

 depression; this is termed the foveola. This is 

 the terminology employed in human ophthal- 

 mology and there seems no good reason why it 

 should not be applied to all vertebrate eyes. 



When the macula is placed on the nasal 

 aspect of the optic entrance it is called the 

 nasal macula or area nasalis; on the temporal 

 side, the temporal macula or area temporalis. 

 In the same way, one speaks of the area or 

 fovea nasalis and fovea temporalis. 



The area centralis presents a thick portion 

 of the retina, where the arrangement of the 

 elements recalls that observed at the level 

 of the "yellow spot" of the human retina. 

 There is in this region an increase of gan- 

 glionic cells and of the receiving or percipient 

 elements — the cones and rods. 



The depth of the fovea? may be regarded as a 

 measure of the sharpness of vision. Slonaker 

 classifies fovea? as deep, medium and shallow. 

 The round fovea is especially "deep" in 

 swift Fliers and Birds of Prey; "medium" to 

 "weak" in most other birds, except that it is 

 "shallow" in the domestic Pigeon, and prob- 

 ably lacking in the Hen. 



Krause is probably right in believing that 

 domestication brings about the disappear- 

 ance of the fovea. 



Both fovese in the bimacular fundi of diur- 

 nal Birds of Prey are structurally much alike, 

 in that they have in them only cones, and only 

 those with yellow droplets. 



Besides this central area (with a fovea) 

 there is often, in Birds, a ribbon- or band-like 



