142 



ELEMENTARY PHYSIOLOGY 



the rings of cartilage are incomplete, and become more and more 

 so as thet ubes subdivide into smaller branches, till in the smallest 

 divisions they disappear altogether. 



The trachea and its branches are all lined internally by a 

 delicate mucous membrane, the inner surface of which is formed 

 of a layer of ciliated cells. These are cells, the free surfaces of 

 which are covered with minute hair-like projections called cilia 

 (Lat. cilium, an eyelash). The cilia, by their continuous vibratile 

 motion, tend to move any foreign matter, which may come in 

 contact with the inner surface of the air passages, towards the mouth. 

 In this way the lungs are kept comparatively free from particles 



of dust, &c., derived from 

 the air we breathe ; and 

 during inflammation the 

 phlegm discharged from 

 the mucous membrane is 

 always urged towards the 

 larynx, from which it is 

 discharged by coughing. 

 Were it not for this pro- 

 vision, it is clear that we 

 should constantly be in 

 danger of suffocation, con- 

 sequent on the accumula- 

 tion of matter in the air 

 passages. 



The lungs are spongy 

 and elastic bags, consist- 

 ing of air-tubes and cells, 

 blood-vessels and elastic 

 tissue. They fill the cavity 

 of the thorax, with the 

 exception of the space 

 occupied by the heart, the 

 great vessels, and the oesophagus (see Lesson XII.), conse- 

 quently their general form coincides with that of this cavity. 



Both right and left lungs are divided by deep depress ons into 

 lobes the right into three, and the left into two. The lobes are 

 again divided by lesser depressions into lobules (see fig. 7). 

 Each lobule is a miniature representation of the whole lung, being 

 supplied with its own systems of air-tubes and blood-vessels. 



The lungs are supplied with dark venous blood by means of 

 the pulmonary arteries which proceed from the right ventricle. 

 These arteries divide and subdivide into smaller and smaller 

 branches, penetrating every portion of the organs, till at last they 



Fig. 132. Groups of Air-cells at the termi- 

 nation of a small Bronchial Tube. 



