SENSES, INSTINCTS, AND INTELLIGENCE 319 



membrane, touch-spots, and taste-spots, there are nerve- 

 endings particularly sensitive to changes of temperature 

 and pressure, and it may be that there is a magnetic sense. 

 But sight and hearing seem to count for most in the bird's 

 intensely active life. 



§ I. The Eye 



The eye may be usefully compared to a camera, the 

 convergence of the rays of light being effected by the lens 

 and the cornea, the sensitised photographic plate being 

 represented by the retina. The densely pigmented choroid 

 outside the retina makes the back of the eye a dark chamber. 

 The firm and tough external layer of the eyeball, the sclerotic, 

 is a supporting structure. The contraction and expansion 

 of the iris diaphragm in front lessens and increases the 

 amount of light that enters the camera. The region in 

 front of the lens and iris is the anterior chamber of the eye, 

 containing aqueous humour. The region behind the lens 

 is the posterior chamber and filled with vitreous humour. 

 The lens is moored by a suspensory ligament made up of 

 many fibres that radiate from the partly muscular ciliary 

 processes which project inwards at the junction of cornea 

 and sclerotic. 



Birds have a very mobile neck, with practically universal 

 freedom of movement, and it is therefore unnecessary that 

 there should be great mobility in the eye. The usual 

 six eye-muscles are reduced and the spherical shape of the 

 eyeball, adapted to ready mobility, is departed from. 

 It often looks as if the eyeball were made up of the parts 

 of two spheres of different radius, a smaller anterior corneal 

 region, usually very convex and often asymmetrically 

 turned towards the nostril, and a larger posterior region 

 turned inwards and backwards. The nearest approach 

 to the spherical shape is seen in the ostrich and in some 

 high-soaring birds of prey ; there is often much flattening, 

 as in swan and parrot ; there is a constriction and elongation 

 of the median portion and a protrusion of the anterior 



