APPLICATION OP THE ARGUMENT. 
21 
hoop, # [Plate III. fig. 1, 2.] surrounding the broadest part 
of the eye; which, confining the action of the muscles to 
that part, increases the effect of their lateral pressure upon 
the orb, by which pressure its axis is elongated for the pur¬ 
pose of looking at very near objects. The other is an ad¬ 
ditional muscle, called the marsupium, [Plate III. fig. 3, 4, 
6.] to draw, upon occasion, the crystalline lens back, and 
to fit the same eye for the viewing of very distant objects 
By these means, the eyes of birds can pass from one ex¬ 
treme to another of their scale of adjustment, with more 
ease and readiness than the eyes of other animals. 
The eyes of fishes also, compared with those of terres¬ 
trial animals, exhibit certain distinctions of structure adap¬ 
ted to their state and element. We have already ob¬ 
served upon the figure of the crystalline 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 
adjusted to near objects, in this respect differing from the 
human eye, as well as those of quadrupeds and birds. The 
ordinary shape of the fish’s eye being in a much higher 
degree convex than that of land animals, a corresponding 
difference attends its muscular conformation, viz. that it is 
throughout calculated for flattening the eye. 
The iris also in the eyes of fish does not admit of con¬ 
traction. This is a great difference, of which the proba¬ 
ble reason is, that the diminished light in water is never 
too strong for the retina. 
In the eel, [Plate III. fig. 5.] which has to work its head 
* The flexible rim, or hoop , consists of bony plates, which in all 
birds occupy the front of the sclerotic; lying close together and overlap¬ 
ping each other. These bony plates in general form a slightly convex 
ring. Fig. 1, but in the accipitres they form a concave ring, as in Fig. 2, 
the bony rim of a hawk. It is a principle in optics, that the rays of light, 
passing through a lens, will be refracted to a point or focus beyond the 
lens, and this focus will be less distant in proportion as the lens approach¬ 
es to a sphere in shape. This principle is very naturally applied to the 
explanation of the use of this apparatus. These scales partly lying over 
each other, so as to allow of motion, will, on the contraction of the 
straight muscles inserted into and covering them, move over each other, 
and diminish the circle of the sclerotica; and thus the cornea, which is 
immediately within the circle made by these scales, must be pressed 
forwards and rendered more convex, from the focus of the eye becoming 
altered, by its axis being elongated. This consequent convexity of the 
cornea renders small objects near the animal very distinct. Without this 
structure a bird would be continually liable to dash itself against trees 
when flying in a thick forest, and would be unable to see the minut 
objects on which it sometimes feeds.— Paxton. 
