SENSES, INSTINCTS, AND INTELLIGENCE 321 



The cornea consists of two lamellae which are movable 

 on one another. A muscle, called Crampton's, runs from 

 a circular ridge on the outer margin of the inner lamella 

 to the margin of the sclerotic ring. The contractions of 

 this muscle flatten the periphery of the cornea and cause 

 the central portion to bulge outwards. Another muscle, 

 called Miiller's, extends from the circular ridge of the cornea 

 into the choroid, and other fibres pass into the ciliary 

 processes which are closely united with the surface of the 

 lens. In a complicated way the various muscles combine 

 in adjusting the focus of the eye for different distances, 

 and in this function birds excel all other animals. The 

 most important point is that this power of accommodation 

 in birds is twofold, partly by altering the lens (as in other 

 Vertebrates), and partly by altering the curvature of the 

 cornea. 



There is strong contractility in the mobile pigmented 

 iris which forms a diaphragm-like membrane in front of 

 the lens, with the pupil in the centre. In other Vertebrates 

 the musculature of the iris is smooth or unstriped, which 

 means relatively slow contraction ; but in birds the muscle- 

 fibres of the iris are striated or striped, which means rapid 

 contraction. In accommodation the iris is pressed firmly 

 by its circular muscles against the front of the lens, and there 

 is, for the time being, no communication between the anterior 

 and the posterior chamber of the eye. 



The important general point is that birds surpass all 

 other creatures in their power of rapid accommodation. 

 With great rapidity and ease they can change the focus of 

 their eyes from that suited for seeing a near object to that 

 suited for seeing something at a distance. As Mr. Beebe 

 says, the bird's eye " in a fraction of time can change itself 

 from telescope to microscope." At one moment the hen 

 is anxiously scanning the sky lest a carrion crow is near ; 

 next second she is inspecting a minute insect larva among 

 the dust at her feet and picking it out for her chickens. 



Besides the musculature for altering the curvature of 

 the lens and the cornea, there is a remakable development 



Y 



