STRVCrrRE OF LUNGS 



301 



wards is released, the box and bag' resume their i'oriiier \ ohuues, and 

 air is expelled. In short the lungs are a form of suction pump or 

 bellows (Fig. 94). 



Structure of Mechanism. 



While the foregoing accoimt of the principle underlying the 

 respiratory mechanism may be taken as substantially correct, it 

 is apt to convey a wrong impression of the details of the mechanism. 



{a) The lungs are not simple elastic 

 bags, but are composed of thousands of 

 little distensible air sacs — forming an 

 elastic sponge-work. 



(6) Each lung is subdivided into lobes, 

 the left lung having two lobes and the 

 right lung three lobes, and is in com- 

 munication with the external air through 

 the trachea and bronchi. The windpipe, 

 or trachea, divides into two bronchi, one 

 of which, with the pulmonary blood and 

 lymph vessels, etc., enters each lung at 

 the root of the lung. The bronchi on 

 entering the lungs undergo repeated 

 branchings, and finally each tiny termmal 

 bronchiole subdivides into a number of 

 alveolar ducts. On these are little ex- 

 pansions, the atria, from which the air 

 sacs or alveoli open. Each bronchial 

 branch is accompanied by a branch of the 

 pulmonary artery, which ultimately breaks 

 up into a fine network of blood capillaries 

 on the walls of the air sacs. The blood 

 from the capillaries is returned to the left 

 side of the heart by the pulmonary veins. 



(c) These complex bags are suspended in and ahnost fill the 

 thoracic cavity. Each lung is enclosed in a membranous sac — the 

 pleura, which, on reaching the root of the lung, bends back from 

 the bronchi and lines the entire internal surface of the chest wall. 

 Each lung has, in its development, pushed into a closed sac, and, 

 carrying the walls of that sac with it, has l^ecn completely 

 enveloped by it. That is, the pleura consists of two layers — an 

 outer, parietal or chest wall layer, and an inner, visceral or lung 

 layer. The surfaces of the two layers are kept moist with lymph. 

 It is important to note that as long as the chest wall is kept intact 

 the pleural cavity is only a cavity in name. The layers of the 



FlCJ. 94. — Model to demonstrate 

 aclion of diaphragm. On pulling 

 the rubber sheet downwards, air 

 enters the lungs and they expand. 



