ACCOMMODATION IN BIRDS 281 



in birds, for it is completely embedded in the sclera. Brucke's muscle 

 inserts, not upon Crampton's, but upon a partition of scleral material. 



The anatomical and physiological divorce of the anterior part of the 

 ciliary muscle from the posterior, together with the especially deep sclero- 

 corneal sulcus of birds, seems designed to promote the cornea-deforming 

 action which we noted was possibly present, though not marked, in rep- 

 tiles. At least, it is in the birds which have the heaviest Crampton's 

 muscles (hawks and owls) that the cornea changes most in shape during 

 accommodation. Again, in diving birds, whose corneae are extra-thick 

 and stiff, and of no optical use under water anyway, Crampton's muscle 

 is all but absent. 



Another, and little-understood, muscular apparatus of bird eyes which 

 may have something to do with accommodation is found in the fundal 

 portion of the chorioid. Here (particularly, it is claimed, in the neigh- 

 borhood of the fovea) there are scattered short, thick muscle cells run- 

 ning rivet-fashion through the thickness of the chorioid. It has been 

 suggested that these, by varying the thickness of the chorioid, serve to 

 adjust very precisely the position of the fovea in accommodation. Their 

 action would be comparable to that of the fine adjustment of a micro- 

 scope, the ciliary muscles being the coarse adjustment. It is as likely that 

 they regulate the blood volume of the chorioid and thus affect vitreous- 

 cavity volume and pressure during accommodatory changes or changes 

 in altitude during flight. In some birds these chorioidal muscle elements 

 are striated, in others smooth, in still others absent; and until these dif- 

 ferences have been studied further and correlated with other avian fea- 

 tures, intra-ocular or extra-ocular, we will have no certainty as to quite 

 what they mean. If they are indeed a micrometer adjustment for accom- 

 modation, they may explain why avian foveal cones are not elongated, as 

 are those of other foveate animals in which such elongation relieves the 

 accommodation of the necessity of being extremely precise (see p. 182). 



Most birds are emmetropic or a little hypermetropic, but the wingless 

 kiwi (Apteryx) is somewhat myopic. This nocturnal bird is reputed to 

 have poor vision at all distances, especially in the daytime; and it un- 

 questionably has the poorest eye, all-round, of any bird. As would be 

 expected, diurnal birds have somewhat more extensive accommodation 

 than any reptiles, the extent of accommodation being related to their 

 feeding habits (see p. 366). Even such ordinary-eyed birds as the domes- 

 tic hen and pigeon have a range of eight to twelve diopters; but the owls 

 have half of this or less. The homer, whose vision is probably better than 



