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THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Fig. 17.13. Lejt, a section through the head of the White-bellied Swallow. Three 

 features are characteristic of birds, the most supremely eye-minded of all verte- 

 brates: the eyeball is relatively the largest among animals (the eye of an ostrich is 

 2 inches in diameter); a vascular nutrient organ, the pecten, is attached to the 

 retina which does not contain blood vessels; the retina, which is the essential, sen- 

 sory layer of the eye, is the most elaborate among animals, and the foveas or areas 

 of clearest vision, two in each eye are the most perfect. In the diagram the lines RT 

 and LT represent rays of light. They are reflected on the foveas from an object in 

 front of the bird and fall on the foveas in each eye creating two-eyed or binocular 

 vision. The lines HN and IN represent rays from objects visible on only one side 

 of the bird and fall only on one fovea creating monocular vision. Right, sections 

 of the retina of a crow and a dog, shown with the same magnification. They empha- 

 size the relatively great thickness of the bird's retina. The visual cells are near the 

 tops of the figures. Light enters from the bottom. (Courtesy, Wood: The Fundus 

 Oculi of Birds. Chicago, The Lakeside Press, 1917.) 



Carotenoids are red and yellow pigments that are most abundant in plants, 

 especially carrots. They can be transformed to vitamin A (Table 11.1) and 

 are stored as such in the liver. Like other substances that have important 

 and common uses, they are plentiful and widely distributed. They are present 

 in light receptors of such great variety as the orange-red light spots of the 

 protozoan Euglena, the eyes of starfishes, squid, and crabs, and the rod and 

 cone cells of vertebrates (Fig. 17.17). Carotenoids are the visual pigments, 

 the purples, violets, and yellows often referred to in connection with the 

 rods and cones, especially of the human eye. They are being found in more 

 and more animals; three of them have recently been extracted by George 

 Wald from the cone cells of chickens (Table 11.1, vitamin A). Similar 

 studies by Wald and others indicate that the perception of light, including 

 color, is basically dependent upon these common pigments. Wald has stated: 



