CRANIAL NERVES AND SENSE-ORGANS 



243 



the lens towards the retina ; in reptiles, birds and mammals 

 by the ciliary (Briicke's) muscle which modifies the shape of 

 the plastic lens. This muscle is very feeble in domestic 

 animals ; in man, apes and beasts of prey it is very powerful, 

 and in birds and reptiles actually " striped " and subject to 

 voluntary control, so that it forms an accessory muscle to the 

 iris. 



For distant vision the ocular arrangements differ greatly in 

 various animals according as the eye is required to see at long 

 distances in the air, or at short distances in the water. The 

 eye of the new-born infant, or the adult man, like the eyes of 

 all non-aquatic Vertebrates, is hypermetropic (long-sighted), a 

 condition dependent upon the length of the axis of the eye, the 



FIG. 121. Eye resting (F) and accommodating (N), magnified five times, after 

 Helmholtz. (Munk-Schultz, Physiologie.) 



curve of the limiting surfaces of the refractive media, and the 

 relative positions of these surfaces. All aquatic Vertebrates 

 have eyes which are myopic in the air, because their almost 

 spherical lens has a high refracting power. Amphibians lie 

 midway between the two. 



The acuity of vision is not solely determined by trans- 

 parency of the refracting media, but also by the sharpness of 

 definition of the image on the retina, its size, and the perceptive 

 sensibility of the nerve elements. The larger the eye the 

 greater is the retinal image and the visual acuity. The appar- 

 ent size of an object does not depend, however, merely upon 

 the magnitude of the retinal image, for all objects in space 

 will project images in a fixed ratio whatever the size of the 



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