38 



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 aerating 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 biids 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 nature of a feather. It is a modification of the 

 skin, just as are the scales 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 true skin. 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. Tlie chief muscle em|ii '''"'l ti'.us 

 is tlie great pectoral, attaciied at one end to the large kcei or ridge 

 of the breast bone, and at the 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 



