116 



coats ; but in all animals that have the eye more removed from 

 a spherical form, as the Cetacea, Fishes and Birds, that mem- 

 brane is supported by hard accessory parts ; or by a greater 

 solidity of texture and a more considerable thickness." 



In a Paper in the third vol. of the Zoological Journal, 

 Wm. Yarrell says, " the eyes of birds are much larger in pro- 

 portion than those of quadrupeds, and exhibit also two other 

 peculiarities." One of these peculiarities is *' a ring of thin 

 bony plates enveloped in the sclerotic coat. Comparative 

 Anatomists do not seem to be agreed as to the means by which 

 birds obtain their powers of vision ; whether by alteration in 

 the form or situation of the crystalline lens, or by both ; either 

 or both of which, the greater quantity of aqueous humour 

 which birds are known to possess, would seem to facilitate, and 

 the existence of a muscle attached to the inner surface of the 

 bony hoop of the sclerotica and inserted by a tendinous ring into 

 the internal surface of the Cornea, as shewn by Mr. Crampton 

 in the Annals of Philosophy for 1813, by which the convexity 

 of the Cornea may be altered, gives a still greater scope of 

 action." He afterwards says, " the external convex form of 

 the Golden Eagle will be found to extend through all the 

 species of every genus of British birds, except the Owls, in all 

 of which it is concave." 



In speaking of the sclerotic bones generally. Dr. Buckland, 

 page 174, of his Bridgewater Treatise, says, " In living animals 

 these bony plates are fixed in the exterior or sclerotic coat of 

 the eye, and vary its scope of action by altering the convexity 

 of the cornea ; by their retraction they press forward the front 

 of the eye and convert it into a microscope ; in resuming their 

 position when the eye is at rest, they convert it into a telescope." 



As regards the form of the bony ring, Blumenbach certainly 

 is not correct when he says, that '* in the accipitres it is con- 

 cave externally ;" that is only true of the nocturnal accipitres, 

 as stated by my friend Wm. Yarrell ; nor is the latter correct in 

 saying that every species of British birds has it convex like the 

 Golden Eagle. The Woodcock, Spoonbill, Caprimulgus, and 

 some others have merely a flat and narrow ring towards the 



