ORNITHOLOGY. 277 



tribes, according to the mode in which they are adapted to take 

 their prey. Of the five senses, sight, smell and hearing are 

 most acute in birds. The crystalline humor of the eye is 

 flat in birds; the vitreous humor is very small. The color of 

 the iris varies in different species, and in many cases is very 

 brilliant. Birds have three eyelids, two of which, the upper 

 and lower, are closed in most of the race by the elevation of the 

 lower one, as may be seen in our domestic poultry. But the 

 third eyelid, or nictitating membrane, forms the most curious 

 apparatus. When at rest it lies in the corner of the eye ; but 

 by the combined action of two muscles which are attached to the 

 back of the sclerotic cjat, (the white,) of the eye, it can be 

 drawn out so as to cover the whole front of the eye-ball, like a 

 curtain, and its own elasticity restores it to its resting place. 

 This, it is said, enables the eagle to look at the sun. 



The rapacious birds seem most remarkable for their length of 

 sight; others, as the swallow tribe, which fly with extraordinary 

 swiftness, have an almost inconceivable quickness of sight. 



The sense of hearing in birds appears to be in general tolera- 

 bly acute, especially in the nocturnal birds of prey, which have 

 what other birds are without, an external cartilaginous ear. 



The sense of smell does not seem to be very highly devel- 

 oped in the birds as a class, but is strong in the vultures. 



Few of them have a tongue which serves as an organ of taste, 

 but some of the swimmers and the parrots generally have one 

 that is soft, thick and covered with papillae, and there can be no 

 doubt that these taste food of a soft or fluid nature, and select 

 that which they like best ; for the most part, however, the tongue 

 is an organ for taking food rather than of taste. The sense of 

 touch is in birds generally very obtuse. 



The dress or plumage of birds is admirable for its fitness to 

 the ends for which it was designed ; for its softness, smooth- 

 ness, compactness, and various hues. The most brilliant col- 

 ors are lavished upon the " winged denizens of the air." This 

 is particularly true of birds of the torrid zones. Those of the 

 temperate zones are not so remarkable for the elegance of their 

 plumage ; but the smaller kinds make up for this defect by the 

 melody of their voices. While the birds of the warmer regions 

 are very bright and gorgeous in their colors, they have scream- 

 ing voices, or are totally silent. The frigid zones, where the 

 seas abound in fish, are stocked with >irds of the aquatic kind far 

 more than any other regions. These usually have a warmer 

 coat of feathers, or they have large quantities of fat lying under- 

 neath the skin, which serves to defend them from the rigors of 

 the climate. 



