THE PHYSIOLOGICAL PRINCIPLES OF VENTILATION 759 



they fail to get rid of effete products properly, substances which have 

 no action on them under the ordinary conditions of temperature become 

 toxic and so forth. A highly abnormal internal environment therefore 

 becomes created around the living tissues of the body. 



The Relationship between the Conditions of Ventilation and Suscepti- 

 bility to Infections. But short of a measurable rise in the temperature, 

 improperly ventilated places cause reactions in the human body that are 

 responsible not only for the discomfort which is experienced, but also for 

 a lowering of resistance to infections. These reactions are due in the 

 first instance to alteration in the temperature differences between the 

 skin and the underlying tissues. Normally, as has been remarked before, 

 this difference maintains at the skin a constant stimulation of the ther- 

 mic nerves, and this stimulation is important in maintaining the tone of 

 the nerve centers. The nerve cells that control the functions of the 

 body do not originate impulses; they only act when other afferent im- 

 pulses arrive at them. There are many varieties of stimuli which may 

 excite these afferent impulses, but none more important than those which 

 excite the heat nerves of the skin. This stimulation depends on the rate 

 at which heat is passing through the sense organs (or receptors), in 

 which these nerves terminate. It is necessary to emphasize that it is 

 the rate of change that acts as the stimulus, and this depends on the dif- 

 ference between the deep and superficial temperatures. When the skin 

 vessels become dilated, so large a volume of blood reaches the surface 

 that this difference becomes slight, and the thermic receptors are not 

 stimulated. There are many practical applications of these principles; 

 thus it is because of stimulation of the thermic skin nerves that cold 

 baths have a bracing effect, that the open-air treatment, as in tuberculo- 

 sis, tones up the body and enables it the better to hold its own against 

 the tubercle bacillus, and that sleeping out of doors is the best tonic for 

 maintaining good health. In the open-air treatment it is true that the 

 body is closely wrapped up that is essential but this does not elim- 

 inate the cooling influence, for not only does the cool air play on the 

 exposed face and hands, in the skin of both of which the thermic nerves 

 are very sensitive, but it acts also on these nerves in the skin, under the 

 clothes, for the clothing merely serves to regulate the rate of cooling. 

 This still goes on very much more than it would with much less clothing 

 in an atmosphere that is stagnant, hot, and humid. Open windows in 

 bedrooms are never so healthy as open-air porches, because there is no 

 draft. It is the draft that is important. Naturally it must be regulated 

 so that it is not restricted to one part of the body only that obviously 

 would introduce conditions to which the body is unaccustomed it 

 must blow equally all over. There is probably no greater fallacy in pop- 

 ular hygiene than that drafts are dangerous. Like all good and desirable 



