ORGANS OF VISION. 91 



overlap each other so as to admit of a slight degree -of motion. 

 These osseous plates are most largely developed in the Owls, as the 

 horned species or Hibous, where they form a cup-shaped ring of 

 remarkable depth, and are usually as long as they are broad. The 

 cornea is in general highly convex, to the greatest degree in the 

 Birds of Prey, and least in the Natatores. The anterior chamber 

 of the eye is very large, and contains an abundant quantity of 

 aqueous humor, so that the iris is very far removed from the cornea. 

 The choroid is provided upon both its surfaces, especially the inter- 

 nal or retinad, with a dense and black layer of pigment. It is 

 attached in front and externally to the sclerotica by a dense white 

 ciliary ligament or zone, from which fasciculi proceed to the iris and 

 largely plicated ciliary processes. These fasciculi, like the fibres 

 of the iris, exhibit under the microscope the transverse striae so 

 characteristic of voluntary muscular tissue. Between the folds of 

 the ciliary ligament is situated the canal of Fontana. The iris 

 presents numerous shades of color, but is never of a metallic hue ; 

 the yellow color of the iris, e. g. in the Owls, being due to the pres- 

 ence of distinct racemoid follicles or cells which contain a fluid fat. 

 The pupil, which can be voluntarily contracted and dilated by the 

 movements of the iris, is always round. The optic nerve perforates 

 the posterior part of the globe of the eye by a slightly excentric elon- 

 gated fissure. The retina presents a well-developed papillary or 

 nervous layer, arid the vitreous humor, which is in considerable 

 quantity, exists in a semi-fluid condition. The variously-shaped but 

 never spherical lens is generally more convex upon its posterior 

 surface. 



A peculiar mechanism is met with in the eye of birds, which 

 has been called the pecten or marsupium. This organ, which is 

 more or less quadrangular, and varies in length and breadth, con- 

 sists of a plicated membrane invested like the choroid (from which 

 it is a production corresponding in structure) with a layer of black 

 pigment, and projects forward from the fissure of entrance of the 

 optic nerve into the vitreous humor, frequently, though not always, 

 reaching as far forward through the apex of its longest fold, as 

 the posterior surface of the capsule of the lens. It is very richly 

 supplied with blood, by an arterial plexus given off from a ra- 

 muscule of the ophthalmic artery, which corresponds to the arteria 

 centralis retinae. The form and number of the plications of the 

 marsupium vary remarkably in the several genera and species, 

 and even in individuals of the latter. In the Ostrich and Casso- 



