140 ANATOMY OF VERTEBRATES. 



terminal line of the optic nerve ; tliey are of a rounded figure in 

 most species, but in the Ostrich and Cassowary they are com- 

 pressed, 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. ^ The folds vary in number, being four in 

 the Cassowary, seven in the Great Horned Owl, eight in the 

 Maccaw, from ten to twelve in the Duck and Vulture, fifteen in 

 the Ostrich, sixteen in the Swan and Stork, and still more nu- 

 merous in the Insessorial Birds, amounting to twenty-eight, 

 according to Soemmerring, in the Fieldfare. 



The exact functions of the marsupial membrane are still in- 

 volved in obscurity. Its position 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 ac- 

 cordingly supposed that it contributed to render more distinct the 

 perception of objects placed in front of the eye. 



Some physiologists have supposed that this black membrane 

 was extended toward 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 exposed in soaring aloft 

 against the blazing sun. Others have considered it as the gland 

 of the \dtreous 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. 



The marsupium may act as an erectile organ, and occupy a 

 variable space in the vitreous humour : when fully injected, there- 

 fore, 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 increased size of the mar- 

 supium ; the contrary effects will ensue when the vascular action 

 is diminished. The nocturnal Apteryx, in which the eye is so 

 small, shows also the exception of the absence of the marsupium. 



The retina is continued from the circumference of the base 

 of the marsupium, and after unfolding its plicte expands into 

 a smooth layer of medullary matter, which seems to terminate 

 at the periphery of the corpus ciliare. In the Owls not more 

 than half the globe of the eye is lined by the retina ; it ceases in 

 fact where the eye loses the spherical form at the base of the 

 anterior cylindrical portion. 



* The Parisian Academicians, who took their description of this part from the 

 Ostrich, first applied to it the name o^ Marsnpinm or Bourse, xl'. 



