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BIRD’S-EYE VIEWS. 579 
cepted differs from all of these hypotheses, and makes out 
the marsupium to be an “erectile” organ. Although no 
muscular fibres have been shown to exist in it, yet it is 
probably capable of expansion and contraction much as if it 
were muscular. It is a highly vascular structure, as we 
have seen; and the increased or diminished turgidity of its 
numerous blood-vessels* would, of course, alter its dimen- 
sions. If it occupies a variable space in the vitreous humor, 
it must affect the position of the lens, and by this means 
change the focus of the eye. This seems to be the most 
satisfactory explanation, both of the design of the marsu- 
pium, and of the mode in which its design is carried out. 
In this view, the organ is marshalled with several others 
that we know contribute to the greatest physiological phe- 
nomenon of a bird’s eye, —the nbid adjustment of focus. 
As anatomists, we have examined the structure, and posi- 
tion, and appearance of the organs that make up a hird’s eye. 
But our study would be to little purpose if it ended here with 
an inspection of dead tissues. We have seen some curious 
things that, perhaps, have afforded us gratification, which is 
well enough as far as it goes; but curiosity is only laudable 
when, disdaining amusement as an ulterior object, it is con- 
tented only with, a higher aim,—instruction. We must 
look, as physiologists, at the operations of the eye, and the 
mode in which its functions are conducted and accomplished. 
All that has gone before is merely to prepare us to question 
intelligently the structures we have examined, and find out 
w they work. 
Eyes are made to see with, of course; but how we see 
with our eyes nobody knows. No one can tell us how an 
adh etl alee 
*It is not a arent, at ht, whence a marsupium gets its numerous vessels, 
Since it is n 1s fete gs tdi eg hy mbrane of the eye, ~ the choroid. Pro- 
fessor 
remarks on this subject: “ piers hes of the ophthalmic Imic artery, —_— 
ea Any vessels of the sorok, per snes with the retine, € g 
i. — the retina, along the whole extent of the oblique 8 
te sclerotic and ai and immediately penetrate the folds ds of the mar 
a riie, upon which they form delicate ramifications.” (Anatomy of ates, 
+ P. 139.) 
