298 



AVES. 



it is rapidly and forcibly depressed; the resistance 

 of the air thus suddenly struck occasions a 

 reaction on the body of the bird, which is 

 thereby raised in the same manner as in leap- 

 ing from the ground. The impulse being once 

 given, the bird folds the wings by bending the 

 different joints, and raises it preparatory to 

 another stroke. 



Velocity of flight depends upon the rapidity 

 with which the strokes of the wings suc- 

 ceed each other. A simple downward stroke 

 would only tend to raise the bird in the air ; 

 to carry it forwards the wings require to 

 be moved in an oblique plane, so as to strike 

 backwards as well as downwards. The turn- 

 ing in flight to the right or to the left is prin- 

 cipally effected by an inequality in the vibra- 

 tions of the wings. To wheel to the right the 

 left wing must be plied with greater frequency 

 or force, and vice versa. 



The outspread tail contributes to sustain 

 the posterior part of the body ; when depressed 

 during a rapid forward flight, the anterior part 

 of the body is raised, and flight retarded ; 

 when the tail is raised the anterior part of the 

 body is lowered. Some birds bend the tail to 

 one side, using it as a rudder when the hori- 

 zontal course of flight is required to be changed. 

 The first launch of the bird into the air is pro- 

 duced by an ordinary leap from the ground, 

 and depends, in some degree, on the length 

 of the legs. Those birds which have very 

 short legs and very long wings, as the Swallows, 

 &c., cannot leap high enough to gain the 

 requisite space for the expansion of their wings, 

 and consequently have much difficulty in raising 

 themselves from the ground, and generally pre- 

 fer throwing themselves from some high point. 

 The manner of flight varies exceedingly in 

 different birds, some dart forward by jerks, 

 closing their wings every three or four strokes ; 

 the Woodpeckers, Wagtails, and most of the 

 small Insessores are characterized by this kind 

 of undulatory motion : other birds, as the Swal- 

 low, Crow, &c. fly smooth and even : the Kite 

 and Kestrel Hawk and the great Albatross some- 

 times appear to buoy themselves in the air with- 

 out any perceptible motion of the wings. 



The rapidity with which a strong Bird of Prey 

 flies in pursuit of his quarry is inconceivably 

 great. The anecdote of the Falcon belonging to 

 Henry IV. King of France, which flew in one 

 day from Fontainbleau to Malta, a distance of 

 1350 miles, is well known, and many similar 

 instances are on record. The flight of a Hawk, 

 when its powers are fully exerted, is calculated 

 at one hundred and fifty miles an hour. The 

 Eider-Duck's usual flight has been ascertained 

 to be at the rate of ninety miles an hour. 



The famous Race-horse Eclipse is said to 

 have gone at the rate of a mile in a minute for 

 a very short distance; but this speed, if it 

 could be continued, would not be half so 

 great as that which many birds put in practice 

 during their long journeys of migration. 



Of the Nervous System. There is a remark- 

 able uniformity in the form and structure of the 

 brain (Jig. 134, a, b, c, d) and medulla spinalis 

 (e,e) in the different orders of birds. These great 



divisions of the cerebro- 

 spinal axis are always 

 readily distinguishable 

 from one another by the 

 greater breadth and glo- 

 bular form of the brain, 

 which is proportionally 

 much larger than in the 

 other oviparous verte- 

 brata. The high degree 

 of development which 

 the spinal cord and 

 cerebellum present, as 

 compared with the cold- 

 blooded Reptilia, has 

 an evident relation to 

 the extraordinary loco- 

 motive powers with which 

 the feathered class is en- 

 dowed . 



In a Pigeon weighing 

 eight ounces with, and 

 seven ounces without its 

 feathers, or three thou- 

 sand three hundred and 

 sixty grains, the cerebro- 

 spinal axis weighs forty- 

 eight grains, the weight 

 of the spinal cord be- 

 ing eleven, and that of 

 the brain thirty-seven 

 grains. 



Of the Brain. The 

 brain of the bird differs 

 from that of the reptile 

 in the superior size of 

 the cerebrum, and the 

 more complex structure' 

 of the cerebellum ; it 

 differs from the brain of 

 a mammal in the smaller 

 size of the cerebellum, 

 resulting from the want 

 of the lateral lobes, and 

 in the absence or rudi- 

 mentary condition of the 

 fornix; and it differs 

 from the brain of every 

 other vertebrate class in 

 the lateral and inferior 

 position of the optic lobes 

 or bigeminal bodies.* 



It cannot be at once 

 distinguished, as Cu- 

 vier asserts, by being 

 composed of six out- 

 ward and visible masses, 

 since the two hemi- 

 spheres, (a, a,) the two 

 optic lobes, ( b, b,) the 

 cerebellum, (c,) and 

 medulla oblongata, (d,) 



* We have lately as- 

 certained that the corpus 

 callosum is wanting in some 

 of the marsupial animals ; 

 its presence is therefore no 

 longer characteristic of the 

 class mammalia. 



(Fig. 134. 



s J 



-Of 



Brain and Spinal Cord 

 of a Goose. 



