168 THE BODY AT WORK 



the lower edge of the cricoid cartilage, which is easily felt in 

 the neck beneath the thyroid cartilage (Adam's apple), to the 

 under side of the arch of the aorta, where it divides into the 

 right and left bronchi. The epithelium which lines the trachea 

 and .bronchi is ciliated. The cilia propel the secretion which 

 accumulates on its surface upwards towards the larynx. The 

 wall of the windpipe is kept open by rings of cartilage which 

 are incomplete behind, where the trachea and oesophagus are 

 in contact. Rings and plates of cartilage also support the 

 bronchi. The bronchi divide and subdivide until their 

 diameter is reduced to about 0-2 millimetre. Each bronchiole 

 then breaks up into a bunch of very thin-walled, elongated 

 infundibula, club-shaped, and with a diameter about five times 

 that of the bronchiole with which they are connected. They 

 may be three or four times as long as they are broad. The 

 wall of an infundibulum is pitted like a piece of honeycomb 

 into shallow chambers the air-cells or alveoli. 



The walls of the air-chambers, or alveoli, are formed of a 

 membrane upon which is spread a network of capillary blood- 

 vessels. The air-chambers are so closely packed together that 

 a common wall separates one chamber from the next adjoining. 

 Minute bloodvessels pierce the partitions which separate the 

 chambers, appearing now on one side of the wall, now on the 

 other. The air-chambers are lined by thin epithelial scales or 

 tiles. The blood in the capillary vessels is separated from the 

 air in the air-chambers by the wall of the capillary ; by 

 a lymph-space, probably rather potential than actual ; and 

 by the epithelial tiles. This covering suffices to prevent 

 the escape both of red corpuscles and of plasma, yet offers 

 very little resistance to the passage of gases from the blood 

 into the air, and from the air into the blood. 



Leucocytes make their way between the tiles, and creep 

 over their internal surfaces, searching for cell debris or foreign 

 matter. Anything that they find they carry to the clumps of 

 lymphoid tissue which occur in the outer wall of the bronchi. 

 In a town- dweller, leucocytes are found in these lymph- 

 thickets, charged with particles of soot. They show droplets 

 of fat and other evidences of degeneration. At other spots are 

 to be seen little collections of soot which have been left behind 

 after the dissolution of the leucocytes which brought them there. 



