314 INTRODUCTION TO ZOOLOGY. 



media of very different densities, and of different degrees of 

 transparency. Conditions have, therefore, to be fulfilled with 

 regard to the eye of birds, which are not required in the best 

 optical instrument of human construction; and, at the same 

 time, it is needful that the focal distance, fitted for near or for 

 distant vision, should be adjusted with a rapidity very different 

 from the "rack and pinion" adjustments of our most skilful 

 opticians. Details connected with this subject would here be 

 out of place, and must be sought for in works of a less ele- 

 mentary character.* 



One obvious peculiarity may, however, be mentioned: birds 

 possess, not two, but three eyelids. The third, termed the 

 nictating membrane, lies in the inner angle of the eye when 

 not in use. By the action of powerful muscles, it can in a 

 moment be swept over the surface of the eye, and then by its 

 own elasticity spring back to its former place. It is mem- 

 branous, and somewhat transparent; and some authors who 

 describe the Eagle as gazing on the sun, assert that he does 

 so by means of the protection which this membrane affords. f 



Smell. The sense of smell in birds has been subjected to 

 various experiments, to ascertain the extent to which it exists; 

 and the development of the olfactory nerves in more than one 

 species has been examined by Professor Owen.J A Vulture, 

 which formed the subject of one of his investigations, was the 

 Turkey Buzzard ( Vultur aura), a bird extremely abundant in 

 Jamaica, where it is known by the familiar name of " John 

 Crow." It feeds on carrion, and its services are considered 

 so valuable, that the killing of one within a certain distance 

 of the principal towns is an offence punishable by fine. The 

 notes of Professor Owen prove the existence in this Vulture of 

 a well-developed organ of smell. The same fact is established 

 by the observations of Mr. Sells. It is to be recollected that, 

 in hot climates, the burial of the dead commonly takes place 

 in about twenty-four hours after death, on account of the 

 rapidity with which decomposition takes place. " On one 



* Jones's Outline, p. 609. Yarrell's Birds, 1st edition, vol. i. p. 11, 14, 

 and 138. 



f The poet thus refers to the popular belief: 



" Nay, if thou be the princely Eagle's bird, 

 Show thy descent by gazing 'gainst the sun." 



KING HENRY VI. Part iii. Act ii. scene 1. 

 J Proceedings of Zoological Society, March, 1837. 



