368 ANIMAL PHYSIOLOGY. 
coarse and fine anatomy of the respiratory organs, we call at- 
tention to the physiological aspects of some points. The lungs 
represent a membranous expansion of great extent, lined with 
flattened cells and supporting innumerable capillary blood-ves- 
sels. The air is admitted to the complicated foldings of this 
membrane by tubes which remain, throughout the greater part 
of their extent, open, being composed of cartilaginous rings, 
completed by soft tissues, of which plain muscle-cells form an 
Fie. 298.—Mold of a terminal bronchus and a group of air-cells moderately distended by 
injection, from the human subject (Robin). 
important part, serving to maintain a tonic resistance against 
pulmonary and bronchial pressure, as well as serving to aid 
in the act of coughing, etc., so important in expelling foreign 
bodies or preventing their ingress. 
The bronchial tubes are lined with a mucous membrane, 
kept moist by the secretions of its glands, and covered with 
ciliated epithelium, as are also the nasal passages, which by 
the outward currents they create, favor diffusion of gases, and 
removal of excess of mucus. The thoracic walls and the lungs 
themselves are covered with a tough but thin membrane lined 
with flattened cells, which secrete a small quantity of fluid, 
