THE STRUCTURE OF THE LUNGS 133 



of an infundibulum sends flattened projections into its 

 interior and thus forms a series of thin partitions by 

 which the cavity of the infundibukim is divided up into a 

 large number of little sacs or chambers. These sacs are 

 the alveoli or air-cells. 



The very thin walls (Fig. 39, B h) which separate these 

 alveoh are supported by much delicate and highly elastic 

 hssue, and carry the wide and close-set capillaries into 

 which the ultimate ramifications of the pulmonary artery 

 pour its blood (Fig. 39, C, D). Thus, the blood contained 

 in these capillaries is exposed on both sides to the air- 

 being separated from the alveolus on either hand only by 

 the very delicate pellicle which forms the wall of the 

 capillary, and the lining of the alveolus. The partitions 

 between the alveoli are covered with extremely thin 

 flattened cells, which may be easily seen in the lung of a 

 young animal but are reduced to almost nothing in the 

 lung of an adult. 



The infundibula are bound together in groups by con- 

 nective tissue to form larger masses termed lobules. 

 These lobules are similarly-bound together in groups to 

 form lobes and the several lobes are united to form a 

 lung. The blood-vessels, nerves and lymphatics of each 

 lung are carried by the connective tissue which binds the 

 whole together. 



The trachea is essentially a tube whose waU is strenc^th- 

 ened and whose bore is kept open by C-shaped hoops or 

 rmgs of cartilage. These hoops lie imbedded in fibrous 

 connective tissue in the outer part of the waU, in which 

 also there is a certain amount of unstriated muscular 

 tissue, running chiefly across the space between the ends 

 of each cartilaginous hoop. The inner surface of the tube 

 IS lined with a mucous membrane, i This consists of 



f»lT^'^ ""^"^ is appUed generaUy to the membranes lining those in- 

 ternal passages of the body which communicate with its surface such 

 .^s the respiratory passages, the alimentary canal, and the bladder 

 Mucous membranes become continuous with the skin at the edge of the 

 openmg on the surface. They derive theu- name from the fact that they 



