244 DR KNOX on the Philosophical Anatomy 



satisfy myself of its existence. The most delicate arterial injec- 

 tions fail in shewing a single branch of a vessel carrying coloured 

 fluids in the zonule of Zinn, or capsule of the lens of birds ; 

 but the marsupium or pecten, passing through the centre of the 

 vitreous humor towards the lens, receives from the central arte- 

 ry a most abundant supply of bloodvessels : a few of them, 

 though they must be very delicate, are seen to pass to the cap- 

 sule of the crystalline *. Hence, it would seem, that the dif- 

 ferent arrangement of the vessels which occurs in the mammalia, 

 and in birds, occasions a difference in structure ; but that the 

 analogies are closely observed. In man, and in some other ani- 

 mals, the vessels intended to nourish the vitreous humor are 

 supplied to it chiefly by those sent from the ciliary processes, 

 from the branches of the central artery of the retina which are 

 distributed to its inner tunic ; and, lastly, though I believe only 

 in the young animal, from that branch of the central artery of 

 the retina which passes through the centre of the vitreous hu- 

 mor. In birds, all these vessels are collected into one large 

 group, and, as it were, projected through the mass of the vitre- 

 ous humor ; the vascularity of the inner membrane of the retina 

 has disappeared, and the zonule of Zinn, so conspicuous for its 

 vascularity and for the complexity of its structure in the mam- 

 malia, is reduced to a mere rudiment, destitute of bloodvessels. 

 We are forced, then, to consider the marsupium as analogous to 

 these organs ; as being in fact their substitute ; its principal, if 

 not its sole function, being to support, nourish and absorb the 



* I am not sure if I rightly understand a passage in M. DE BLAINVILLE'S work 

 entitled, " Principes de 1'Anatomie Comparee." He there states, that he has seen a 

 bloodvessel entering the marsupium in the eye of the casuary ; but several of the 

 preparations exhibited to the Society, shew that sixteen or seventeen distinct vascu- 

 lar trunks may be counted in the marsupium of the common domestic fowl. 



