200 HANDBOOK OF PHYSIOLOGY. 



however, that the organism after a time adapts itself to such a vitiated 

 atmosphere, and that a person soon comes to breathe, without sensible in- 

 convenience, an atmosphere which, when he first entered it, felt intoler- 

 able. Such an adaptation, however, can only take place at the expense 

 of a depression of all the vital functions, which must be injurious if long 

 continued or often repeated. 



This power of adaptation is well illustrated by the experiments of 

 Claude Bernard. A sparrow is placed under a bell-glass of such a size 

 that it will live for three hours. If now at the end of the second hour 

 (when it could have survived another hour) it be taken out and a fresh 

 healthy sparrow introduced, the latter will perish instantly. 



It must be evident that provision for a constant and plentiful supply 

 of fresh air, and the removal of that which is vitiated, is of far greater 

 importance than the actual cubic space per head of occupants. Not less 

 than 2000 cubic feet per head should be allowed in sleeping apartments 

 (barracks, hospitals, etc.), and with this allowance the air can only be 

 maintained at the proper standard of purity by such a system of venti- 

 lation as provides for the supply of 1500 to 2000 cubic feet of fresh air 

 per head per hour. (Parkes.) 



The Effect of Respiration on the Circulation. 



The heart and great vessels being situated in the air-tight thorax, 

 are exposed to a certain alteration of pressure when the capacity of the 

 latter is increased ; for although the expansion of the lungs during in- 

 spiration tends to counterbalance this increase of area, it never does so 

 entirely, since part of the pressure of the air which is drawn into the 

 chest through the trachea is expended in overcoming the elasticity of 

 the lungs themselves. The amount thus used up increases as the lungs 

 become more and more expanded, so that the pressure inside the thorax 

 during inspiration, as far as the heart and great vessels are concerned, 

 never quite equals that outside, and at the conclusion of inspiration -is 

 considerably less than the atmospheric pressure. It has been ascertained 

 that the amount of the pressure used up in the way above described, varies 

 from 5 to 7 mm. of mercury during the pause, and to 30 mm. of mer- 

 cury when the lungs are expanded at the end of a deep inspiration, so 

 that it will be understood that the pressure to which the heart and great 

 vessels are subjected diminishes as inspiration progresses. It will be 

 understood from the accompanying diagram how, if there were no lungs 

 in the chest, but if its capacity were increased, the effect of the increase 

 would be expended in pumping blood into the heart from the veins, but 

 even with the lungs placed as they are, during inspiration the pressure 

 outside the heart and great vessels is diminished, and they have therefore 

 a tendency to expand and to diminish the intra-vascnlar pressure. The 

 diminution of pressure within the veins passing to the right auricle and 



