230 



EYE 



A.ch 



however, brown, and attains brighter tints with maturity occasionallj- 

 in the male only : for instance, yellow in the males of Ploceus, but 



brown in the females : 

 greyish -brown in the 

 females and young of 

 the Golden Oriole, and 

 carmine-red in the old 

 males. ^ 



The iris contains a 

 sphincter and a dilator 

 muscle. The sphincter 

 consists of concentrical 

 fibres which constitute 

 a considerable portion 

 of the whole iris, while 

 the fibres of the dilator 

 are arranged in a radi- 

 ating way. Those of 

 the former are supplied 

 by branches from the 



N.O. 



Horizontal Section through the Bye of a Bird-of- 



Prey. About twice the natural size. (After H. MiillerO 

 A.ch. Anterior chamber; P.ch. Posterior chamber; C, 

 Cornea ; Ch. Chorioid ; Co, Conjunctiva ; c.-ni. Crampton'.s oculomotory Or third 

 muscle; ^, Iris: L, Lens • P, P^-cten : ^^-O- Optic nerve; -^ ^f ^.^.^^j^j ^^^ 



iJ, Eetina ; Sc. Sclerotic, with embedded osseous ring. -r _ t-.- i • 



and are in Birds, in 

 opposition to the Mammalia, under the control of the will. The 

 dilator muscle is .supplied by sympathetic nerve fibres. Both 

 sorts of nerve fibres enter the posterior wall of the eye, ascend 

 between the chorioid and sclera, and supply these parts together 

 Avith the iris and the ciliary muscle. The shape of the pupil, 

 when fully dilated, is round in all birds ; when partly contracted 

 it is either likewise round in most birds, or obliquely oval as in 

 some Gallinsei. 



The cor])us ciliare is that part of the chorioid which covers the 



^ Numerous notes concerning the colour of the iris in reference to age and sex 

 have been published by Th. A. Bruhin in Zoolog. Garten, 1870, pp. 290-295. A 

 curious observation by the late Mr. J. H. Gurney is recorded in Mr. Soutlnvell's 

 continuation of Stevenson's Birds of Ntrrfolk (iii. p. 207). On removing a living 

 male Pochard {Fuligula ferina) from one pond to another, while the bird was held 

 in the hand the ordinary cherry-colour of its irides was seen to be replaced by 

 yellow, hardly, if at all, tinged with red. When this bird was released, the 

 normal colour was gradually but speedily resumed. In this species it ■will be 

 remembered the red iris is peculiar to the male, that of the female being of a dark 

 brown. 



The four nearly-allied species of the genus Hypsipctes inhabiting Madagascar 

 and the Mascarene Islands differ, according to the observation of Sir Edward 

 Newton {Orn. Miscell. ii. p. 52, pi. xlii.), in the colour of their irides, which in H. 

 madagascarknsis are bright red, in H. horhonicus whitish, in H. olivaceus (of 

 Mauritius) orange, and in H. crassirostris (from the Seychelles) dark red. 



