14 bird's-eye views. 



the iris through which we look (the transparent crystalline 

 lens offering no obstruction to our view) directly into and 

 across the posterior chamber of the eye, and see the black 

 pigment on the choroid behind. Albino animate have pink 

 eyes, because the coloring matter of the choroid is wanting, 

 and the hue of the blood in its numerous fine vessels ap- 

 pears. And even if we look into a normal eye with the 

 ophthalmoscope, we have a reddish instead of a black field 

 of vision. The pupil takes its name from a very pretty 

 conceit. On looking straight at it, our image is reflected to 

 us, only so diminished that we are transformed into pigmies. 

 We find an expression of the same thing in other languages 

 beside our own. In Spanish, the liliputian photograph is 

 called "ninacita del ojo ;" which means "little eye-baby." 



But to return from this digression to the iris, which has 

 been all the time nervously quivering at our neglect. It is 

 essentially similar in structure to the choroid, being a deli- 

 cate tissue of fibres and vessels interlacing in every direc- 

 tion ; but it has, in addition, a structure that is regarded as 

 muscular. The iridian muscles are mainly disposed in two 

 ways ; there is a circular set running around, and a radiating 

 set that pass across from the inner to the outer border. By 

 means of these, which are mutually antagonistic, the iris is 

 contracted and expanded, and its aperture — the pupil — 

 correspondingly varied in size. In mammals, the movement 

 of the iris appears to be automatic, and to depend upon the 

 stimulus of light ; and they are not so great, as a general 

 rule, as in birds. In the latter, they are extraordinary, not 

 only m degree, but in the rapidity with which they may be 

 executed. Although birds' irides respond primarily, and 

 perhaps chiefly, to the action of light, their movements seem 

 to be partly, at least, subject to the will, and therefore volun- 

 tary. These conditions of mobility in the iris relate directly 

 to such exigencies as, for examples, the owl meets with in 

 the daytime, or the eagle encounters in his flight towards 

 the sun. 



