190 SPECIAL PHYSIOLOGY. 



tended by special muscles which draw those processes forwards. In the flying 

 Fishes, the so-called flight is accompanied by an impetus taken from the water 

 by the agency of the tail, and of the long powerful pectoral fins, which latter 

 are then spread out in the air, so as to look like wings ; but they have only 

 very feeble muscles at their base, and they merely perform a parachute action, 

 and so sustain the animal for a distance of many feet in the air before its 



fravity again accomplishes its descent into the water. Flying-fish have been 

 nown to rise fifteen or twenty feet from the surface of the water, but the usual 

 height is not more than three feet ; they may remain suspended in the air about 

 half a minute, and thus pass through a distance of even 300 feet. 



In Insects, the mode of flight is explicable on similar principles to those 

 which regulate it in Birds ; but here, also, as was mentioned in comparing the 

 locomotion on land, of the Annulosa with that of the Yertebrata, the mus- 

 cular or moving apparatus is placed within the passive levers on which it acts, 

 instead of outside them. The wings of insects are variously constructed, and 

 present various sizes and forms ; they are horny and membranous, in the 

 beetles ; soft, and feathered with microscopic scales, in the moths and butter- 

 flies ; thin and glassy-looking, in the flies and dragon-flies ; short, in the ear- 

 wigs and house-flies ; long and narrow, in bees and wasps ; broad and full, in 

 butterflies ; and enormously elongated in the dragon-flies. Sometimes they 

 are only two in number (diptera) ; sometimes the anterior ones are converted 

 into protective cases or elytra (beetles) ; but more commonly they are four in 

 number. These wings, however different in character, are invariably attached 

 to the sides of the thoracic segments, above the proper limbs or legs ; they are 

 moved by powerful muscles lying inside the thorax, that part of the body of 

 an insect being developed proportionally to its powers of flight. The base of 

 the stiff framework of each wing projects into the interior of the thorax by a 

 sort of process or spur ; and the muscles act upon this spur, those which draw 

 it downwards raising the wing, and the far more powerful ones, which draw 

 the spur upwards, acting in the downward stroke ; so that the muscular force 

 is applied in the opposite direction to that in which it acts in the bird or bat. 

 The rapidity, duration, and character of the movement performed by different 

 insects on the wing, depend on the area of their wings, on the number of 

 strokes made in a second, and on the character of those strokes, whether rapid 

 and continuous, as in the dragon-fly, or slower, and more interrupted and 

 fluttering, as in the butterfly. Insects, considering their size, fly with much 

 greater rapidity than birds ; the dragon-fly, for example, flies more rapidly 

 than the swallow ; this insect has also much greater control over its organs of 

 flight, and can execute a greater variety of movements in the air, even than 

 the most agile bird. 



In the Amphibia, amongst the Yertebrata, and in the Mollusca, Molluscoida, 

 Annuloida, Ccelenterata, and Protozoa, there are no examples of flying species. 



The organization of birds is entirely, and in every part, directly adapted to 

 flight. First, their biped position in standing and walking, leaves the upper 

 limbs entirely free for locomotion in the air. In the standing posture, the 

 body of the bird is generally raised forward to bring the centre of gravity over 

 the feet, excepting in many swimming birds, as the duck, and others. In flight, 

 the body is usually held more horizontally, and the centre of gravity lies very 

 far forward, a position favorable to that mode of locomotion. In walking, the 

 axis of motion is placed far back, at the hip-joints, but in flight, forward, 

 through the shoulders ; and this change in the seat of motion, requires differ- 

 ent compensatory changes in the position of the body. The length and free 

 motion of the neck, also render the adjustability of the centre of gravity in 

 flight much more easy. The concentration of weight forward in the trunk is 

 accomplished by the muscular masses being chiefly situated there ; the limbs 

 contain the tendons only. The absolute weight of the animal is also diminished 

 as much as possible, in regard to its size and strength, by various conditions, 

 such as the extreme lightness of construction of its skeleton and feathers, the 

 expansion of their solid matter, and the presence of air in the bones and quills. 

 The large size of the lungs in birds, the presence of air-cavities in the body, 

 and even in the bones, the rapidity and energy of their respiratory movements, 

 their consequent high temperature, and the rarefaction of the contained air, 



