286 IMPORTANCE OF FREE VENTILATION. 



oxygen, produced by the burning of gas-lights, lamps, or 

 candles. 



337. Hence we see the great importance of providing for 

 free ventilation, wherever large assemblages of persons are 

 collected together, even in buildings that seem quite adequate 

 in point of size to receive them ; and much of the weariness 

 which is experienced after attendance on crowded assemblies 

 of any kind, may be traced to this cause. In schools, facto- 

 ries, and other places where a large number of persons remain 

 during a considerable proportion of the twenty-four hours, it 

 is impossible to give too much attention to the subject of 

 ventilation ; and as, the smaller the room, the larger will be 

 the proportion of carbonic acid its atmosphere will contain, 

 after a certain number of persons have been breathing in it 

 for a given time, it is necessary to take the greatest precaution 

 when several persons are collected in those narrow dwellings, 

 in which, unfortunately, the poorer classes are compelled to 

 reside. Even the want of food, of clothing, and of fuel, are 

 less fertile sources of disease than insufficient ventilation; 

 which particularly favours the spread of contagious diseases, 

 on the one hand by keeping-in the poison, and thus concen- 

 trating it upon those who expose themselves to its influence ; 

 and, on the other, by obstructing the elimination of the waste 

 matter from the system, the presence of which in the blood 

 renders it peculiarly liable to be acted-on by all poisons 

 having the nature of " ferments." 



338. When the quantity of carbonic acid in the air accu- 

 mulates beyond a certain point, it speedily produces suffocation 

 and death. This is occasioned by the obstruction to the flow 

 of blood through the capillaries of the lungs, which takes 

 place when it is no longer able to get rid of the carbonic acid 

 with which it is charged, and to absorb oxygen in its stead. 

 The general principle to which this stagnation may be referred 

 has already been noticed ( 280). Now, as all the blood of 

 the system, in warm-blooded animals, is sent through the 

 lungs before it is again transmitted to the body, it follows 

 that any such obstruction in the lungs must bring the whole 

 circulation to a stand. The functions of the nervous system 

 are directly dependent upon a constant supply of arterial 

 blood (Chap, x.) ; and, accordingly, as this supply becomes 

 progressively diminished in quantity and deteriorated in qua- 



