THE RESPIRATORY EXCHANGE 



483 



association of infants and young children in large numbers, has 

 had in many respects untoward results, has often increased the 

 infectious diseases to which children are liable, has diminished 

 the responsibility of mothers for the care and training of their 

 children, and has imposed prematurely upon young children an 

 artificial and unhealthy routine of life. 



The mucus secreted by the nose appears to possess properties 

 antagonistic to the growth of bacteria, and thus acts both as a 

 chemical and mechanical defence. There are limits to the effec- 

 tiveness of this nasal sieve, for the dwellers in cities and the workers 

 in coal-mines have numerous particles of carbon in their lungs. 

 It is true that in these cases the objection may be raised that the 

 particles are inhaled through the mouth ; it has been shown, ( 4 ) 

 however, that in rabbits, which breathe only through the nose, 

 the mucous surfaces of the turbinate bones act at first as an 

 effective filter during the inhalation of air heavily laden with 

 particles of dust. If the experiment be continued the mucous 

 membrane becomes covered and can retain no more dust ; there 

 is a second line of defence in the pharynx and larynx, but even 

 that is overcome if the exposure to the dense dust be prolonged. 

 A similar condition obtains in the case of miners. Haldane, 

 Martin, and Thomas ( 5 ) have shown that the alarming prevalence of 

 so-called miners' phthisis among the Cornish miners is to be attri- 

 buted to the permanent injury of the lungs produced by the stone- 

 dust which is inhaled in large quantities by the miners, especially 

 those who work with rock drills. The impairment of the respiratory 

 functions reacts upon the general health and predisposes to tuber- 

 culosis of the lungs ; thus the death-rate from diseases of the lungs 

 among the tin miners of Cornwall is about four times as high as 

 that for all occupied males in England and Wales. 



The expired air varies in composition according to the rate 

 and depth of respiration ; this is shown by the following analyses 

 made by Speck. 



