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and a peculiar valvular arrangement for propelling that fluid 
vigorously through the body. Moreover in addition to their lungs, 
birds possess a singular provision of air receptacles within the 
body, and these are connected with a series of cavities, also filled 
_ with air, which occupy the interior of most of the bones. These 
cavities serve not only to give lightness to the bird’s body, but they 
also assist the lungs in aeratiug the blood, so that birds may be 
said to possess a kind of double respiratory system, but this I shall 
refer to more particularly later on. As bids exceed mammals in 
the activity of their breathing and circulatory systems, so also 
they possess a higher degree of animal heat, their temperature 
ranging from 2 to 14 degrees, F. higher than that of man. This 
high temperature is maintained by the bird’s admirable feather- 
clothing which being a non-conductor, effectually serves to guard 
against any sudden variations of temperature in the air to which 
its body is exposed during its rapid and extensive flights, as well as 
tempering the usual radiation from the body. 
Since then the possession of feathers is characteristic of a bird, let 
us enquire into the natureofafeather. It is a modification of the 
skin, just asare thescales of the feet and claws of the toes. Feathers 
and hairs, scales and claws are all produced out of, and are modifi- 
cations of the cells of the superficial layer of the skin called the 
epidermis, and of the deeper layer, the cutis vera, or trueskin. As 
one might expect in the forelimb of a creature specially organised for 
suspension in, and progression through the air, it is found that the 
muscles, as well as the bones and joints, of the bird’s wing become — 
much modified, as compared with the corresponding parts of other 
animals, With all our scientific knowledge and mechanical con- 
trivances, we have never yet succeeded in constructing a flying 
machine. It is a very remarkable fact that Nature has not only 
long ago solved this problem, but that she has done so in several 
ways. The flight of an insect, of a bird, of a bat, is equally perfect 
in its way ; but in each case the result is attained by very different 
modifications in the skeletal and muscular apparatus. The prin- 
cipal resistance that a flying animal has to work against is its 
weight ; that is the force of gravity which, proportionately to its 
mass, tends to draw it down vertically towards the earth; hence 
the muscles which are largest and strongest in a bird are those 
which pull down the wing against the air, thereby raising the 
body and overcoming its weight. The chief muscle employed thus 
is the great pectoral, attached at one end to the large keel or ridge 
of the breast bone, and atthe other end to the ‘‘ humerus” or 
‘‘arm bone.” This great pectoral is generally the largest muscle 
in the bird’s body, and in fact often equals in bulk all the other 
muscles put together. The anterior limbs do not touch the 
