ANATOMY OF THE FUNDUS ORGANS IN BIRDS 



23 



The pectinate image seems to be displaced 

 by abrupt movements in the vitreous mass, 

 disturbances due probably to contractions of 

 the ocular muscles, external and internal. 



As Kalt says, the pecten is a membranous 

 organ covered by a black pigment attached to 

 the optic nerve and projecting a variable 

 distance into the vitreous body. This organ, 

 studied and described for the first time by 

 Perrault, in 1876, received later the name 

 comb (peigne). The term "comb" is not 

 exact since there are no separate teeth; per- 

 haps "fan" better describes the appearance 

 of this organ. German authors still call it 

 the "fan" (Fiicher). 



Except in the Ostrich, where it has the form 

 of a cone inserted into the optic nerve and 

 divided into two parts by a sort of white 

 partition, the pecten represents a triangular 

 or rectangular pigmented lamina, whose plane 

 is directed down and out. 



Generally it does not extend into the 

 vitreous beyond the intercalary zone of the 

 sclerotic, and it is only exceptionally, as in 

 the Goose, the Swan, the Stork, etc., that it 

 reaches the crystalline lens. The comb is 

 made up of a lamina folded upon itself and 

 producing, in a transverse section, a markedly 

 zig-zag effect. The number of plications varies 

 from 5 to 30, averaging 16, as in birds of prey 

 and Gallinaceae. The writer and Slonaker 

 counted 18 folds in the English Sparrow. 



Among the Owls, the length of the comb is 

 5 mm. at the base; its height, 4 mm. In the 

 Stork the base is 13 mm. long; it is 7 mm. high. 



Kalt believes the pecten to be essentially a 

 vascular organ. It is composed of a network 

 of capillary vessels of unequal dimensions 

 spread out in two or three superimposed 

 planes, whose mesh may vary in form and 

 dimensions. In the Goose the vessels are 

 nearly all parallel; the anastomoses are rather 

 rare and the meshes acquire great length. 

 The thin connective tissue, which serves as a 

 support, contains free pigment. The entire 

 organ is enclosed in an envelope of endo- 

 thelium. 



Kalt further thinks that the pectinate 

 blood-supply is derived from the external 

 branch of the internal carotid, which, imme- 



diately after leaving the temporal, forms a 

 large plexus. The branches from this plexus 

 cross the sclerotic and re-unite in a single 

 trunk, which corresponds to the central ar- 

 tery of the retina in mammifers. This trunk 

 extends to the base of the comb, where it 

 gives off several ascending vessels, that pene- 

 trate the latter and form a large capillary 

 plexus. The venous blood is returned by a 

 large choroidal vein which perforates the 

 sclerotic a short distance towards the middle 

 of the comb. Note, also, that it makes at 

 the level of the ocular groove several variable 

 anastomoses with the plexus of posterior 

 ciliary arteries. See Figure 13. 



According to Beauregarde the pecten is 

 allied to the choroid; it appears after the 

 fourth day in the embryo of the Hen. 



The function of the pecten has from time 

 to time been variously interpreted. It has 

 been considered particularly as an erectile 

 organ capable of displacing the crystalline 

 lens and thus taking part in the accommo- 

 dation of the eye. Beauregarde, using the 

 ophthalmoscope, found displacements of the 

 organ and rapid vibrations synchronous with 

 the movements of the nictitating membrane. 

 But he noticed, above all, that the comb is 

 placed so as to intercept the rays coming 

 from the front and from above; that is to say, 

 those rays that reach the two eyes simulta- 

 neously. In catching them, the pecten is 

 supposed to suppress momentarily, in this 

 way, binocular vision, a condition which is 

 necessary for the more perfect use of monocu- 

 lar vision. 



No doubt the pecten occasionally serves as 

 a screen to protect the retina against the raj's 

 of the sun. Observation of the attitude of 

 the Hen's head, when exposed to the sun, is 

 favorable to this supposition, as, also, the 

 slight development of this organ among 

 nocturnal birds. 



Another hypothesis, which seems plausible, 

 attributes to the comb the role of an organ of 

 nutrition for the vitreous, and makes of it an 

 appendage of the ciliary body. It has also 

 been suggested that it is an organ of excre- 

 tion, charged with maintaining the intraocular 

 tension. 



