AMPHIBIOUS BIRDS 441 



spectacles, we may compare that of the sea-duck with a 'contact lens'! 

 Captive American mergansers have been observed to pursue their trout 

 and salmon prey by sight. They can evidently accommodate sufficiently 

 under water to give themselves a near point within ten feet, for they 

 unerringly follow the movements of their victims at that distance. 



The size of the cornea has been reduced by the conversion of a zone 

 of its substance, near the limbus, into opaque sclera-like material. In the 

 cormorant a further similarity to some aquatic mammals is seen in the 

 thickening of the sclera at the sclerocorneal junction. In all of these birds 

 the scleral ossicles are particularly heavy, so thick as to have marrow 

 cavities within them like those of the hawks (Fig. 112, p. 280) ; and div- 

 ing ducks have thick corneas. These thickenings possibly stiffen the wall 

 of the eyeball against the shock of immersion, perhaps only support it 



Fig. 149 — Accommodation in amphibious birds. 



a, anterior segment of cormorant, Phalacrocorax sp., in relaxation. Redrawn, modified, from 

 von Hess, b, same as a, in accommodation, showing action of the powerful iris sphincter. 

 Note that th« fibers of the pectinate ligament are taut and that the spaces of Fontana, 

 behind them, have become dilated by the pull of the iris. 



cp- ciliary process; is- iris sphincter; pi- pectinate ligament; so- scleral ossicle. 



against the unusual pull of the augmented accommodatory apparatus. 

 One part of the latter is at a low ebb in these underwater swimmers : 

 /. e., Crampton's muscle. Its function being chiefly to shorten the radius 

 of curvature of the cornea as an aid to accommodation for near objects 

 (p. 281), and there being no point to any manipulation of a refractive 

 surface which is just 'not there' under water, this muscle is reduced in 

 some amphibious species and is absent in others. There is none in the 

 cormorants, it may be lacking or small in loons and auks, and it is small 

 in the diving fuliguline ducks (as compared with the non-diving anatine 

 ones). On the other hand the muscle of Briicke (Fig. 112) is massive in 

 cormorants, stronger in diving than in non-diving ducks; and in cormo- 

 rants and in the gannet (Morus bassana) it is most exceptional in con- 

 taining circular fibers like those of the human muscle of Miiller. 



