74 THE INDIVIDUAL ORGANISM 



tern. This system consists essentially of (1) a large expanse of moist, 

 thin epithelium, permeable to oxygen and carbon dioxide ; (2) air passages 

 through which the outside air reaches this epithelium; and (3) a breathing 

 mechanism to provide continual renewal of the air in contact with the 

 epithelial surface. 



Air enters the respiratory system through the mouth or nose and, 

 crossing the food passage in the pharynx, enters the upper end of the 

 trachea, or windpipe, where the larynx, or voice box, a strong cartilaginous 

 cylinder, contains the vocal cords. The lower end of the trachea extends 

 into the thoracic cavity to a point somewhat below the level of the first 

 pair of ribs and there branches into the right and left bronchial tubes. 

 Each bronchial tube leads into a lung, in which it branches repeatedly into 

 smaller and smaller tubes (the bronchioles) until the minute terminal 

 branchlets end in expanded air sacs (alveoli) of thin epithelial tissue. Each 

 lung may be roughly compared to a huge bunch of grapes; the stem and 

 its branches correspond to the bronchial tubes; the grapes, to the air 

 sacs. In the lung, of course, all these structures are hollow, and the bron- 

 chial tubes show a much more intricate and compounded branching. In 

 addition to the structures just described, each lung contains an abundant 

 supply of blood vessels, and is closely covered externally by a thin, moist, 

 elastic membrane, the pleurum. Each lung is suspended by its root — the 

 point where the main bronchial tube enters its medial surface — in a 

 cavity lined by a second (outer) layer of pleural membrane. 



The thoracic cavity is enclosed above and on the sides by the body wall. 

 Its floor is formed by the diaphragm, a dome-shaped muscular partition 

 that separates the thoracic and abdominal cavities. The cavity is divided 

 into right and left sides by (1) a central, connective tissue framework that 

 supports the esophagus, the trachea, and a number of blood vessels and 

 (2) the pericardium, which contains the heart. 



In breathing, the capacity of the thoracic cavity is markedly changed 

 by the movements of its walls. When its muscles contract, the diaphragm 

 becomes much less dome-shaped, and the floor of the thorax is consider- 

 ably lowered. At the same time, contraction of the intercostal muscles 

 lifts the anterior ends of the downward sloping ribs, increasing the depth 

 of the thorax from front to back, and rotates them slightly outward, 

 increasing the breadth of the thorax from side to side. The space between 

 the inner and outer pleural membranes is airtight, and when the outer 

 pleural membrane is pulled outward and downward by its tight connec- 

 tion to the diaphragm and body walls, the elastic inner pleural membrane 

 must likewise expand. Since the inner pleural membrane is closely at- 

 tached to the air sacs of the lungs, they too must enlarge and so draw in 1 



1 To be more accurate, air is forced by atmospheric pressure into a space wherein 

 the air pressure has been lowered. 



