146 TEXT-BOOK OF ZOOLOGY 



contraction and expansion, these movements would considerably affect 

 the action of the organs of flight, which require fixed points of support. 

 These movements, accordingly, must be prevented without the bird's 

 respiration being interfered with. A man cannot breathe easily when 

 his arms are performing some strenuous labour, e.g., lifting a weight. In 

 birds, however, by means of the air-sacs, the respiratory function can 

 proceed uninterruptedly during flight. Owing to the bird's rapid flight 

 through the air, and the resistance of the latter, air is forced from without 

 through the lungs into the air-sacs (inspiration). (This forcible entrance 

 of air into the lungs is experienced in ourselves when walking against a 

 strong wind.) By the strokes of the wings, on the other hand, the air- 

 sacs which lie between the shoulder-joint and the pectoral muscles are 

 compressed, and the air contained within them pumped out through the 

 lungs (expiration). 



This also explains why a bird can travel at heights in which no mammal 

 could exist for any length of time. Birds have been observed flying at 

 heights of as much as 40,000 feet (nearly eight miles). In consequence of 

 the enormous speed at which birds fly in these altitudes (the carrier 

 pigeon traverses 66 feet, and even more, per second, and the swift about 

 three times that distance), the air in the air-sacs is at a high pressure, so 

 that respiration does not proceed at the pressure of the surrounding medium, 

 (Why must a flying bird move rapidly ? Compare the speed of so-called 

 " bad " flyers with that of the fastest mammals. Why do " bad " flyers 

 — e.g., domestic fowls — always fly close to the surface of the earth — i.e., 

 in the denser layers of the atmosphere ?) 



Their manner of breathing also explains why birds never " get out of 

 breath" (like a mammal), even during their most rapid flights. Birds 

 such as swallows, which have been careering about for hours, will settle 

 down quietly without the least acceleration being noticeable in their 

 breathing. During rest, and all other movements except flight, respiration 

 is carried on, as in mammals, by means of alternate enlargement and 

 contraction of the thorax. But, as the lower segments of the ribs are 

 ossified and no diaphragm is present, the means by which these changes 

 are produced are somewhat different. The illustration on p. 140 shows 

 that the two portions of each rib are not in one direction, but meet at 

 an angle. When this angle is increased by muscular action, the lower 

 portions of the ribs being straightened out, the sternum is necessarily 

 lowered. The cavity of the body is thus increased, and the lungs must 

 take in air (inspiration). When the angles of the ribs are again decreased, 

 the cavity becomes smaller, and the air is driven out of the lungs 

 (expiration). 



The upper end of the trachea commences, as in mammals (see Part I., 



