AVES. 



305 



Parrot, Turkey, Cassowary, Stork, Goose, and 

 Swan, that the marsupium seems absolutely to 

 adhere to the capsule of the lens ; but in many 

 other birds, on the contrary, it 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 hu- 

 mour. 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 

 forward the lens. The researches of Bauer* 

 have shewn that 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 which it is almost exclusively com- 

 posed. 



The form of the marsupium varies in differ- 

 ent 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 

 Cassowary. The plicae of the membrane are 

 perpendicular to the terminal line of the optic 

 nerve ; they are of a rounded figure in most 

 species, but in the Ostrich and Cassowary they 

 are compressed, and so far inclined from the 

 plane of the membrane, that their convergence 

 towards its extremity gives it a resemblance to a 

 close-drawn purse.f The folds vary in num- 

 ber, being four in the Cassowary, seven in the 

 Great Horned Owl, eight in the Goose, from 

 ten to twelve in the Duck and Vulture, fifteen 

 in the Ostrich, sixteen in the Stork, and still 

 more numerous in the Insessorial Birds, 

 amounting to twenty-eight, according to Soem- 

 merring, in the Fieldfare. 



The exact functions of the marsupial mem- 

 brane are still involved in obscurity. Its po- 

 sition is such that some of the rays of light 

 proceeding from objects laterally situated with 

 respect to the eye must fall upon and be 

 absorbed by it ; and Petit accordingly supposed 

 that it contributed to render more distinct the 

 perception of objects placed in front of the eye. 

 The theory originally proposed by Sir Everard 

 Home,t which attributed to the marsupium the 

 office of retracting the lens for the purpose of 

 distant vision by its muscular contraction, is 

 opposed by the numerous examples in which 



* Philosophical Transactions, 1822, p. 76. 



t The Parisian Academicians, who took their de- 

 scription of this part from the Ostrich, first applied 

 to it the name of Marsupium or Bourse. The origi- 

 nal description is as follows : " De cet entonnoir 

 (the termination of the optic nerve) sortoit une 

 membrane plissee, faisant commeune bourse qui abou- 

 tissoit en pointe vers le bord du Christallin le plus 

 prochain de 1'entree du nerf optique. Cette bourse, 

 qui estoit large de six lignes par le bas, a la sortie 

 du nerf optique, et qui alloit en pointe vers le haut, 

 estoit attachee par sa pointe aubord du Chrystallin, 

 par le moyen de la membrane qui le couvroit du 

 coste de 1'humeur vitree, et qui couvroit aussi toute 

 la bourse q\ii estoit noir mais d'un autre noir que 

 n'est celuy de la choroide." Duvernoy, in * Me- 

 moires pour servir a 1'Hist. Nat. des Animaux/ 

 p. 375. 



J Croonian Lecture, PhiL Trans. 1796. 



VOL. I. 



it does not extend to the chrystalline, and by 

 the manner of its attachment in those cases in 

 which it does; since, as in these the mar- 

 supium adheres to the side of the chrystalline, 

 it can only move it obliquely. 



Some physiologists have supposed that this 

 black membrane was extended towards the centre 

 of the eye, where the luminous rays are most 

 powerfully concentrated in order to absorb the 

 excess of intense light to which birds are ex- 

 posed in soaring aloft against the blazing sun. 

 Others have considered it as the gland of the 

 vitreous humour, and that, as this fluid must 

 be rapidly consumed during the frequent and 

 energetic use made of the visual organ by 

 Birds, it therefore might require a superadded 

 vascular structure for its reproduction. 



We are inclined to consider the marsupium 

 as an erectile organ, adapted to receive a vary- 

 ing quantity of blood, and to occupy a variable 

 space in the vitreous humour ; when fully in- 

 jected, therefore, it will tend to push forward 

 the lens, either directly or through the medium 

 of the vitreous humour, which must be dis- 

 placed in a degree corresponding to the in- 

 creased size of the marsupium ; the contrary 

 effects will ensue when the vascular action is 

 diminished. From the analogy of other struc- 

 tures introduced by Supreme Wisdom into the 

 mechanism of organized bodies, it may reason- 

 ably be supposed that the marsupium is not 

 limited to a single function. 



The retina is continued from the circumference 

 of the base of the marsupium, and after forming 

 a few slight folds expands into a smooth layer 

 of medullary matter, which seems to terminate 

 at the periphery of the corpus ciliare. In the 

 Owls, as Haller has observed, not more than 

 half the globe of the eye is lined by the retina ; 

 it ceases in fact where the eye loses the sphe- 

 rical form at the base of the anterior cylindrical 

 portion. 



The humours of the eye no less correspond 

 to the peculiar vision of the bird, and the rare 

 medium through which it is destined to move, 

 than the shape of the globe and the texture of 

 its coats. 



The aqueous humour is extremely abundant, 

 owing to the extent of the anterior chamber 

 gained by the convexity of the cornea, and 

 its refractive power must be considerable in the 

 higher regions of the atmosphere. The mem- 

 brane inclosing it can be more readily demon- 

 strated in birds than in most mammals, espe- 

 cially where it adheres to the free edge of the 

 iris. The large size of the ciliary processes 

 may have the same relation to the repro- 

 duction of the aqueous, as the marsupium is 

 supposed to have with reference to the vitreous 

 humour. 



The chrystalline lens is remarkable for its flat- 

 tened form, especially in the high-soaring 

 Birds of Prey ; it is also of a soft texture, and 

 is without any hard nucleus, as in the eyes 

 of Fishes and Reptiles. In the Cormorant 

 and other birds which seek their food in 

 water, the chrystalline is of a rounder figure, 

 and this is peculiarly the case in the near- 

 sighted Owls which hunt for prey in obscure 



