Genus Falco, 185 



plied with air by an orifice at the situation which answers to the 

 front of the great trochanter, the large bones forming the pelvis, 

 the vertebra?, sternum, furcula, clavicles, scapulae, and even the 

 ribs, are all furnished with apertures for the admission of air, 

 supplied from the various cells of the abdomen, sides, and thorax. 

 This distribution of air to the bones does not seem however to be 

 absolutely necessary for flight, since the young birds of our sum- 

 mer visitors appear to perform their first autumnal migration with 

 perfect ease and celerity, at an age when the cavities of their 

 bones are filled with marrow. 



The various characters of the feet are too obvious to require 

 particular notice. 



The extraordinary powers of vision which birds are known to 

 exercise beyond any other class of animals, are in no genus more 

 conspicuous than in that of Falco. Their destination, elevating 

 themselves as they occasionally do into the highest regions, and 

 the power required of perceiving objects at very different dis- 

 tances, and in various directions, as well as the rapidity of their 

 flight, seem to render such a provision necessary. The eyes of 

 birds are much larger in proportion than those of quadrupeds, and 

 exhibit also two other peculiarities. The one is the marsupium, 

 a delicate membrane arising at the bottom of the eye, and termi- 

 nating at or near the edge of the crystalline lens : the other is 

 a ring of thin bony plates, enveloped by the sclerotic coat. 

 Comparative anatomists do not seem to be agreed as to the 

 means by which birds obtain their power of vision, whether by 

 an alteration in the form or situation of the crystalline lens, or 

 by both, either or both of which, the greater quantity of aque- 

 ous humour which birds are known to possess, would seem to 

 facilitate. The existence of a muscle attached to the inner sur- 

 face of the bony hoop of the sclerotica, and inserted by a tendi- 

 nous ring into the internal surface of the cornea, as shewn by Mr. 

 Crampton,* by which the convexity of the cornea may be altered, 

 gives a still greater scope of action, since with two, or at the ut- 

 most three varieties of powers, the sphere of distinct vision may be 

 indefinitely extended. Whether the five species called the 



* Annals of Philosopliy for March, 1813. 



