Dis . BIRD’S-EYE VIEWS. 
would be a perfect sheath were it not split lengthwise. 
Through this slit, and through a corresponding one in the 
choroid membrane, a fold or fluting of the nerve rises up, 
finally attaining the inside of the eye. The retina spreads 
out from all along the sides and extremities of this fold. 
Only one other structure remains to be described—the 
crowning anatomical peculiarity of a bird’s eye. This is 
the marsupium, or pecten (f). Though attached at one end 
to the optic nerve, it is not a part of the nerve at all, nor 
composed of nervous tissue. It is a very vascular mem- 
brane, most like the choroid in texture, and likewise painted 
black. When fully extended, it is seen to be of an oblong 
or rectangular shape; when lying naturally in situ, it is 
much drawn up, and its sides are transversely wrinkled or 
plicated. It is suspended in the vitreous humor, running 
obliquely forwards a great part, or the whole of the way, 
from the end of the optic nerve to the crystalline lens. In 
the former case it appears attached anteriorly to some of the 
lamine of the hyaline ; in the latter to the capsule of the lens. 
Behind, it is always fastened to the optic nerve. It is called 
the “marsupium,” because it does not in the least resemble 
a purse or pouch; and the “pecten” because it does not 
look anything like a comb. Anatomists have not agreed 
upon what to consider as the function of this organ, nor up 
the quo modo of its operation. Some have thought that it. 
absorbs the superfluous rays of light that must often enter 
the eye, because it is blackened with pigment. One who 
adhered to this belief went further, considering that, from 
its eccentric position, it absorbs mainly oblique rays, whieh 
being taken away, objects placed in direct rays may be more 
plainly perceived. Some, again, have regarded it, aoei 
sequence of its vascular structure, as the organ that secretes, 
or aids the choroid in secreting the vitreous humor; aN addi- 
tional apparatus being needed for the elaboration of this 
fluid, because it is used up so fast in the rapid and incessant 
movements of the eye. But the theory now generally a0 
