340 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1916. 



objects it has been found that there is, with the passage backward of 

 the posterior surface of the cornea, the transference of fluid from the 

 anterior chamber. This is shown by injecting methylene bhie into 

 the anterior chamber and stimuhiting the nerves of accommodation, 

 then noting the course of the fluid. 



Admitting then that there is a transference of fluid from one 

 chamber to another to maintain an unvarying intraocular pressure, 

 some governor must be present to effect this quick interchange, and 

 it is believed that the pecten acts in this way. In support of this 

 theory it can be shown that in high-flying birds, birds of rapid flight, 

 birds of pre}^ where the eyes have to be accommodated to extremely 

 rapid alteration of focus, the pecten is well developed. It is, on the 

 other hand, comparatively small in nocturnal birds. Against this 

 theory it may be stated that reptiles, or some reptiles, possess a 

 pecten, and in these animals the above conditions hardly exist. The 

 important point is that the presence of this large pecten creates a 

 large blind area in the eye, and as it is heavily pigmented all light 

 falling on it is naturally absorbed. It explains to some extent the 

 constant shifting of the head when a bird is on the watch, as the 

 visual field is considerably limited, the portion obstructed being 

 toward the upper outer field of vision. Before leaving the retina it 

 should be mentioned that the presence of oil globules in this layer 

 has been known for a long time. These globules are colored red and 

 yellow and are found only in birds. They appear to exert no effect 

 on color vision, as they are in no way identical in composition with 

 the visual purple or sensitizing substance. 



The numerous fibers from the endings of the rods and cones col- 

 lect to form the optic nerves. The nerve from each eye converges 

 and meets at what is known as the optic chiasma, where they unite 

 and again separate. In all animals where binocular vision takes 

 place, or to be more correct, where there is total binocular vision, 

 there is partial decussation of the fiber. Those fibers leading from 

 the right half of the right eye pass to the right side of the brain, 

 while the fibers from the left side of the right eye cross over at the 

 chiasma to the left side of the brain. 



The amount of decussation varies accordingly with the power 

 of binocular vision. In some animals where partial binocular vision 

 is possible, though not usual, as in the horse and some rodents, only 

 a few fibers do not decussate. In animals incapable of any binocular 

 vision complete decussation takes place. This latter condition is 

 found in birds, or nearly all birds, the fibers entirely crossing over 

 at the chiasma. One must first get a grasp of the true meaning of 

 binocular vision to appreciate the difference between pure binocular 

 vision and seeing the same object with both eyes. If we hold a piece 

 of paper between the eyes so as to view, say, a red area with the 



