THE MECHANISM OF RESPIRATION 277 



expanded with air, but the blood is pumped in and out of them by 

 the movement of the ribs. During flight, the wing movement insures 

 adequate respiration. The air-sacs play a great part in flight and 

 extend the leverage with which the muscles act, at the same time 

 keeping the body light. Opening the air-sacs in the resting bird causes 

 no upset of breathing. In the flying bird, an immediate dyspnoea 

 is induced, and flight becomes impossible. It has also been shown that 

 for proper heat regulation of the body the air-sacs are necessary. 



In mammals, the gaseous interchange is effected by the movements 

 of the sternum, ribs, and abdomen, drawing air into and expelling it 

 from the air-tight chest, or thorax (see later, p. 284)., 



In man, the respiratory tract may be said to consist of the following 

 parts : - -' 



X -,". : ' ! 



1 . The Nose. The respiratory portion of ,the nose consists of tfee 

 inferior meatus and the lower portion of the superior meatus. These 

 parts are covered by a very vascular ciliated mucous membrane, 

 which provides a mucous secretion. Thick hairs, or vibrissse, project 

 into the respiratory airway, and guard the entrance .against insects. 

 Air taken in through the nose is warmed and moistened, and, mop 

 important still, injurious particles of dust and bacteria are removed 

 from it. The mucous membrane is extremely sensitive to changes 

 in the atmosphere. When the air playing round the head is cool 

 and relatively dry, the mucous membrane is firm and moist, and a 

 good airway is insured; when the air is stagnant, warm, and humid, 

 the mucous membrane becomes engorged, wet with secretion, and 

 boggy. It is not the cold outside atmosphere of winter, but the 

 warm infected air of crowded rooms, which causes the epidemics of 

 " cold in the head." The nose also acts as an accessory resonating 

 chamber in speech, as the term talking " through the nose ' testifies. 

 To produce this the airway through the nose is greatly diminished. 



2. The Pharynx. Air is conducted through the upper part of the 

 pharynx into the larynx. Its shape and movement play a large part 

 in determining the quality of the voice. Here and in the tonsils 

 there is much lymphoid tissue which possibly guards from infection 

 by inhaled bacteria. 



3. The Larynx. This specialized part of the respiratory tract in 

 which the voice is produced is dealt with later (see p. 739). 



4. The Trachea, or windpipe, which divides into the two as 

 bronchi ; these divide into smaller divisions, known as bronchial tubes, 

 which in their turn divide into bronchiales, ultimately opening into 

 the essential distensible elements of the lungs, the funnel-shaped 

 infundibuli, and the alveoli. 



The trachea is provided with ciliated epithelium, and is wet with 

 mucus secreted by the glands of its mucous membrane. In this 

 mucus any entering dust or bacteria are caught, to be carried upwards 

 by the cilia and away from the lungs. The walls of the larger of 

 these passages (the trachea and bronchi) are provided with cartilage. 



The bronchial tubes are supplied with smooth muscle innervated 



