ORGAN OF SIGHT IN BIRDS. 139 



the interior of the eye, instead of a round disc, as in Mammalia, 

 a Avhite narrow streak, from the extremities and sides of which 

 the retina is continued. Branches of the ophthahiiic artery, dis- 

 tinct from the vessels of the choroid, and homologous Avith the 

 arteria centralis retince, enter the eye betAveen the laminae of the 

 retina, along the whole extent of the oblique slit above mentioned, 

 and immediately penetrate the folds of the marsupial membrane, 

 upon which they form delicate ramifications. These vessels are 

 shown in fig. 58, representing the excised marsupium unfolded 

 and spread out. 



The marsupium is lodged like a wedge in the substance of the 

 vitreous humour, in a vertical plane, directed obliquely forward. 

 In those species in which the marsupium is widest, the angle 



58 



Jliiipfc. 



ararsiipium of the eye of a Rook, unfolded, xxvii-. 



next the cornea reaches the inferior edge of the capsule of the 

 crystalline ; but where it is narrow, the whole anterior border is 

 in contact with the same point. This contact is close in some Birds, 

 as the Vulture, Parrot, Turkey, Cassowary, Stork, Goose, and 

 Swan ; but in other Birds the marsupium does not extend further 

 than two-thirds of the distance from the back part of the eye, and is 

 attached at its anterior extremity to some of the numerous laminae 

 of the hyaloid membrane which form the cells for the lodgment 

 of the vitreous humour. In these cases the marsupium can have 

 no influence on the movements of the lens, unless it be endowed 

 with an erectile property, and be so far extended as to push for- 

 w^ard the lens. There is no muscular structure in the marsupium ; 

 and its changes of form, if such occur in the living bird, must be 

 effected by changes in the condition of the vessels of Avhich it is 

 almost exclusively composed. 



The form of the marsupium varies in different Birds ; it is 

 broader than it is long in the Stork, Heron, Turkey, and Swan ; 

 and of the contrary dimensions in the Owl, Ostrich, and Casso- 

 wary. The plica3 of the membrane are perpendicular to the 



