44 HUMAN PHYSIOLOGY CHAP. 



about four and one half inches long and three fourths of 

 an inch in diameter. The cartilages of the trachea are 

 in the form of small C-shaped rings held in place by 

 muscles and connective tissue. The trachea divides at 

 its lower extremity into two smaller air passages, called 

 the right and left bronchi, going to the two lungs. Each 



of these gives rise to 

 many still smaller 

 passages, called bron- 

 chial tubes, which get 

 lighter and finer until 

 they end in the air 

 sacs. The cartilage 



FiG. 17. Showing how a bronchiole ends in has gradually become 

 air sacs. (After Schafer.) , . ^. 



thinner, the rings 



have disappeared, the muscle fibers have slowly faded 

 out, and even the delicate mucous membrane has be- 

 come so thin that air can easily pass through it into the 

 small blood vessels of the lungs. 



Thus we see the course through which the air is led 

 from the atmosphere into the blood vessels of the lungs. 

 How it gets to the tissues we shall study later. 



Get a windpipe from the meat market. 



1. Cut out one of the rings of cartilage and clean off all the 

 muscle and connective tissue. Compare the cartilage with bone. 



2. Cut off a portion of the lining, which can easily be seen when 

 the windpipe is cut open. It consists of mucous membrane. 



48. The Lungs. The lungs are two large, pinkish, 

 spongy masses that completely fill the chest (thorax), 

 except that part which the heart occupies. The chest 



