A Treatise on the Eye/Book 2/Chapter 14
CHAP. XIV.
Of the Marsupium Nigrum, peculiar to Birds.
Sect.
Pag.
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| 1. | This Membrane described, | 247 |
| 2. | Is muscular, but covered with more or less of a black Pigment, in proportion as Birds fly more or less high, | 248 |
| 3. | It is sometimes called the Pecten, from its Resemblance to a Comb in some Animals, and in the Magpy, and some other Birds, it plainly appears musculous towards its Bottom, | 248 |
CHAP. XIV.
Of the Marsupium Nigrum, or[errata 1] Black Purse, peculiar to Birds.
Sect. 1. THIS Part, which is peculiar to Birds, and never to be found in other Animals, takes its name from its Form, being nothing but a Membrane in Form of a Purse, which arises from the Entry of the optic Nerve, and passes thro' the vitreous Humour to its Insertion in that Part of the Edge of the Crystalline which is next the great Canthus. Thus it is described by the French Academists, and, in particular, by Mons. Perrault, in his Mechanique du Animaux; from whom I have caused it to be copied, at Fig. 7. Plate I. which represents the Half of the Globe of an Ostrich's Eye; in which A is the crystalline Humour, B the optic Nerve, and C the black Purse attached above to the Crystalline, and below to the optic Nerve. But, in some Birds, I have found this Membrane of a rhomboidal Figure, agreable to the Account given of it by Derham, in his Phisico-theology, and Dr. Petit, in the Memoires de l'Academie Royale, ann. 1720.
§ 2. This Membrane is always covered. with a black Pigment, which is of a more intense Colour than either that of the Uvea, or Choroides; yet, if it be cleaned and washed, it appears to be composed of muscular Fibres not unlike the Ligamentum Ciliare; as the learned Mons. de la Hire long ago observed. It is remarkable, that, in proportion as Birds naturally fly more high, and by that Means require a more piercing Sight, this Bag is always more black; for our domestic or tame Birds, which either do not fly, or do not fly high, such as Hens, Geese, &c. have this Part not near so black, as Eagles, and other Birds of Prey.
§ 3.[errata 2] Mr. Derham calls this Membrane the Pecten, because of its Resemblance to a Comb in some Animals; and has also observed its Structure to be very like that of the Ligamentum Ciliare; and in the Eye of a Magpy, and some other Birds, which he examined, he tells us, he could plainly perceive it to be musculous towards the Bottom.