32 NATURAL THEOLOGY. 



surrounding the broadest part of the eye, which 

 confining the action of the muscles to that part, in- 

 creases the eftect of their lateral pressure upon the 

 orb, by which pressure its axis is elongated for the 

 purpose of looking at very near objects. The other 

 is an additional muscle, called the marsupium, to 

 draw, on occasion, the crystalline lens hack, and to 

 fit the same eye for the viewing of very distant ob- 

 jects. By these means, the eyes of birds can pass 

 from one extreme to another of their scale of ad- 

 justment, with more ease and readiness than the 

 eyes of other animals. 



The eyes o^ fishes also, compared with those of 

 terrestrial animals, exhibit certain distinctions of 

 structure, adapted to their state and element. We 

 have already observed upon the figure of the crys- 

 talline compensating by its roundness the density 

 of the medium through which their light passes. 

 To which we have to add, that the eyes of fish, in 

 their natural and indolent state, appear to be ad- 

 justed to near objects, in this respect diflfering from 

 the human eye, as well as those of quadrupeds and 

 birds. The ordinar}^ shape of the fish's eye being 

 in a much higher degree convex than that of land 

 animals, a corresponding difference attends its mus- 

 cular conformation, viz., that it is throughout cal- 

 culated ^oY flattening the eye. 



The iris also in the eyes of fish does not admit 

 of contraction. This is a great dift'erence, of which 

 the probable reason is, that the diminished light in 

 water is never too strong for the retina. 



