THE PHYSIOLOGICAL PRINCIPLES OF VENTILATION 761 



because of the normal protective influences which exist on these surfaces. 

 So long as the blood supply is normal, these protective influences are 

 adequate to protect the body from invasion, but if this should become cur- 

 tailed, then the bacteria become active and set up pathological processes. 

 Evidence favoring this view has been obtained by several recent investi- 

 gators by finding that the blood supply of the upper respiratory passages 

 becomes decidedly curtailed when the surface of the body is cooled. For 

 example Leonard Hill and Muecke some years ago examined with a spec- 

 ulum the mucous membranes of the nose under various conditions, par- 

 ticularly out of doors, and in rooms which were ventilated and heated to 

 an average degree. Out of doors the mucosa was pale and taut, and when 

 touched by a probe did not show any pitting. This is the normal con- 

 dition. Indoors it was common to find the membrane decidedly swollen, 

 flushed with blood and covered with thick secretion, and when a probe 

 was pressed on it a depression resulted lasting for some time. In one 

 case that was frequently examined during these observations there was 

 a deflected septum which only partly blocked the nasal passage on one 

 side when the person was outside, but which did so completely under un- 

 favorable conditions of ventilation. It is this swelling of the nasal 

 mucosa and probably of that of the cavities which extend upward from it 

 on to the forehead that causes the sense of stuffiness and probably also 

 the headaches which are common in crowded, overheated places. 



The conditions found to bring about these changes with greatest cer- 

 tainty were when the feet were cold and the air round the head was 

 warm, conditions which are just exactly the opposite of those obtaining 

 out of doors. Here the head is usually more quickly cooled than the feet 



scause convection currents of cool air play around it freely, whereas 

 lext the ground the air is more stagnant. Besides, if the sun is shining. 



ie earth becomes heated by absorbing the heat. The temperature as 



igistered by a thermometer, either wet or dry bulb, may be the same 

 at the feet as at the head. It is not this that counts, however, it is the 

 rate of cooling which is dependent, mainly, on the movement of the air. 

 Now in a poorly ventilated room, such for example as one heated by a 

 stove, or even by radiators, and in which there is no movement of air, 

 the feet become colder than the head, and it is under these conditions 

 that the nasal membranes become swollen. It ought to be emphasized 

 that the cause for these changes is not cold feet alone. It is the com- 

 bination of cold feet and hot head. Out of doors, it is well known, that 

 any one may stand with cold feet for hours without any risk of catching 

 cold, but then the head is really cooling as fast as the feet, because of 

 .convection currents. 



The ideal system of warming a room is to supply radiant heat near 



