niRDS. o 



and because the body of air does not lie so directly under the 

 wing as they rise. 



In order to move the wings, all birds are furnished with two 

 very strong pectoral muscles, which lie on each side of the breast- 

 bone. The pectoral muscles of quadrupeds, aretriding in com- 

 parison to those of birds. In quadrupeds, as well as in man^ 

 the muscles which move the thighs and hinder parts of the body 

 ai« by far the strongest, while those of the arms are feeble : but 

 in birds, which make use of their wings, the contrary obtains ; 

 the pectoral muscles, that move the wings or arms, are of enor- 

 mous strength, while those of the thighs are weak and slender. 

 By means of these, a bird can move its wings with a degree of 

 strength, which, when compared to the animal's size, is almost 

 incredible. The flap of a swan's wing would break a man's leg ; 

 and a similar blow from an eagle has been known to lay a man 

 dead in an instant. Such, consequently, is the force of the 

 wing, and such its lightness, as to be inimitable by art. No 

 machines, that human skill can contrive, are capable of giving 

 such force to so light an apparatus. The art of flying, there- 

 fore, that has so often and so fruitlessly been sought after, must. 

 It is feared, for ever be unattainable ; since as man increases 

 the force of his flying machine, he must be obliged to increase 

 its weight also. 



In all birds, except nocturnal ones, the head is smaller, and 

 bears less proportion to the body than in quadrupeds, that it 

 may more readily divide the air in flying, and make way for the 

 body, so as to render its passage more easy. Their eyes also 

 are more flat and depressed than in quadrupeds ; a circle of 

 iimall plates of bone, placed scalewise, under the outer coat of 

 the organ, encompasses the pupil on each, to strengthen and de- 

 fend it from injuries. Besides this, birds have a kind of skin, 

 "ailed the nictitating membrane, with which, like a vail, they 

 can at pleasure cover their eyes, though their eye-lids continue 

 open. This membrane takes its rise from the greater or more 

 obtuse corner of the eye, and serves to wipe, cleanse, and pro- 

 bably to moisten its surface. The eyes, though they outwardly 

 appear but small, yet separately, each almost equals the brain ; 

 whereas in man the brain is more than twenty times larger 

 than the orbit of the eye. Nor is this organ in birds less adapt- 

 ed for vision 1)y a particular expansion of the optic nerve, which 



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