168 ELEMENTARY PHYSIOLOGY less. 



is a large, thin-walled tube placed inside the thorax and 

 communicating with tlie lymphatic vessels which lie in 

 the abdomen, and, further, it is plentifully supplied with 

 valves. At inspiration the reduction of pressure on the 

 outside of the duct draws lymph up into it from the 

 abdominal lymphatic vessels. At expiration, the lymph 

 cannot pass down again, owing to the valves in the duct, 

 and is therefore sent on towards the junction of the duct 

 with the venous system. Hence the respiratory move- 

 ments on the whole are a not unimportjint aid to the 

 onward flow of lymph (see p. 92). 



13. Ventilation. — In the case of breathing the same 

 air over and over again the deprivation of oxygen, and 

 the accumulation of carljonic acid, cause injury, long 

 before any .signs of even dyspncjea are observed. Under 

 these circumstances uneasiness and headache arise when 

 less than 1 per cent, of the oxygen of the air is replaced 

 by other matters ; the symptoms in this case however are 

 due not so much to the diminution (jf oxygen or the 

 increase of carbonic acid, as to the poisonous effects of 

 the various organic matters present in expired air which, 

 though existing in minute quantities, have a powerfully 

 deleterious action. It need hai'dly be added that the 

 persistent breathing of such air tends to lower all kinds 

 of vital energy, and predisposes to di.sease. Hence the 

 necessity of sufficient air and of ventilation for every 

 human being. 



The object of ventilation is to prevent the accumulation 

 of these organic impurities (p. 151). Since the organic 

 matter does not admit (jf direct estimation, the percentage 

 of carbonic acid in the air is usually taken as an in- 

 direct measure of its amount. Air which has been 

 fouled by breathing is injurious if it contains more than 

 ■05 per cent, of carbonic acid. Knowing the amount of 

 air passed through the lungs in one hour and the amount 

 of carbonic acid it contains (p. 152), calculation easily showa 



