BIRD’S-EYE VIEWS. 575 
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 animals 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 “niñacita 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 
Tule, as in birds. In the latter, they are extraordinary, not 
only in 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 
__ tobe partly, at least, subject to the will, and therefore volun- 
7 + 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 
sun, ; i 











