50 INTRODUCTION. 



assisted in discovering their prey by the sense of smell. See 

 forwards an anecdote of the Eagle related by Mr. Brookes. 



While the touch, taste, and smell, of Birds generally, are 

 certainly not of the first order, their sight is extremely 

 acute. The Hawk, and others of the Falcon genus, can, at 

 a considerable distance, discern an animal, a lark, or a 

 mouse, upon the ground, and pounce upon it with celerity 

 and certainty. 



Anatomists have, it is said, observed in the eye of Birds a 

 particular expansion of the optic nerve, which renders the 

 impression of visible objects more vivid and distinct. To 

 protect the eye, and, perhaps, also to moderate its extreme 

 sensibility, this organ is furnished in many birds with what 

 is called a nictitating membrane, with which the bird can, 

 at will, cover the pupil of the eye while the eyelids remain 

 open ; and hence the Eagle, and some other birds, are 

 enabled to bear, by the assistance of this covering, the 

 strongest light of the sun. 



Birds have neither epiglottis, diaphragm, urinary bladder, 

 nor scrotum. 



The lungs, which are two red, oblong, spongy bodies, 

 attached in the thorax chiefly to the spinal column, are 

 not divided into lobes ; they are covered with a membrane, 

 or pleura, which communicates by many openings with 

 large vesicles or air bags, that are dispersed over the ab- 

 domen as well as the thorax. By these, birds can, at plea- 

 sure, render their bodies more buoyant, and thus ascend to 

 a considerable height, or skim along in the air with a celerity 

 that far outstrips the swiftest steed. The cavity of the 

 thorax of birds is much larger in proportion than that of 

 other animals, much of which is not filled with the lungs, 

 but with air. This, and the thin porous nature of their 

 bones, many of which are filled with air instead of marrow, 



