bird's-eye views. 17 



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 marsuinum, 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 

 laminse 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 upon 

 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, which 

 being taken away, objects placed in direct rays may be more 

 plainly perceived. Some, again, have regarded it, in con- 

 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 ac- 



(3) 



