92 AVES. 



wary the folds are acutely angular like those of a fan, though in 

 other cases they are generally rounded off. Nocturnal birds have 

 in general the fewest number of folds, e. g. the Owls from 5 to 6, 

 the Goatsuckers 5. In the greater proportion of birds from 14 to 15 

 occur. The folds are most numerous in the Passeres, where they 

 amount from 16 to 18, and 22, or even 28, as in Corvus. In Diurnal 

 birds of Prey, there are from 14 to 16 ; in the Gallinae 16 to 18, and 

 in most Palmipedes only 9 to 12. The pecten in the Ostrich is pro- 

 vided with 15 or 16, in the Cassowary with 4 or 5, and with 18 folds 

 in the Humming-bird. 



The function of the marsupium is unknown. It can not in any 

 way contribute, as has been suggested by some anatomists, by the 

 alternate contraction and dilatation of an erectile tissue to alter and 

 adjust the focal distance of the lens within the eye ; this power, which 

 is possessed to a considerable extent by birds, being probably ef- 

 fected by the conjoined action of the muscular and very mobile 

 structures of the iris and ciliary ligament, by which the position of 

 the lens may be shifted. The whole form of the eyeball is very 

 admirably adapted for the exercise of this function, through the large 

 size of its anterior chamber, and the quantity of aqueous humor con- 

 tained therein. 



A remarkable exception to the great uniformity of structure which 

 the eye exhibits throughout the present class, is furnished by that 

 singular bird of New Zealand, the Apteryx australis. The marsu- 

 pium is here entirely wanting (and so far as is known this is the 

 only instance of the absence of that organ among Birds), a condition 

 which may be probably associated with the perfectly nocturnal habits 

 of this species, and its limited range of locomotion. The optic nerve 

 enters the eye by a small round opening, and the globe of the eye is 

 in proportion to that of other birds of very small dimensions ; the lens 

 also is small, and very convex. 



The eye of Birds is moved by four recti and two oblique muscles ; 

 the trochlea or pulley for the tendon of the superior oblique muscle 

 is however wanting, and all the muscles are proportionally very short. 

 All birds have three eyelids ; a superior which is the shortest ; 

 an inferior the largest and most moveable palpebra, provided with 

 a tarsal cartilage, and a third situated in the anterior angle of the 

 eye, which is very moveable, and called the nictitating membrane. 

 The orbicularis palpebrarum muscle, which is most strongly de- 

 veloped in the Apteryx, is inserted into the tarsal cartilage, and 

 when it acts draws the lower eyelid in the direction upward. The 



