RESPIRATION. 287 



level, no respiratory impulses can occur until the oxygen tension of the 

 blood reach that level. This idea must now be modified, if not given up, 

 in face of the experiments, e.g., those of Hering, on cats' blood during 

 apncea, which have shown that animals in a condition of apnoea may 

 have less and not more oxygen in their blood than in a normal state, 

 although the carbonic anhydride is less. One view now taken of the 

 cause of apnoea is that by rapid inflations of the lungs impulses pass up 

 by the vagi, by means of which inspiration is after a while inhibited ; 

 another view is that by the repeated stimulation of the centre by vagus 

 impulses which result in rapid respiratory movements, anabolism is at 

 last arrested. Apnoea is with difficulty produced, if at all, when the 

 vagi are divided. 



Effects of Vitiated Air. Ventilation. As the air expired from 

 the lungs contains a large proportion of carbon dioxide and a minute 

 amount of organic putrescible matter, it is obvious that if the same air 

 be breathed again and again, the proportion of carbonic dioxide and 

 organic matter in it will constantly increase till it becomes unfit to 

 breathe; long before this point is reached however, uneasy sensations 

 occur, such as headache, languor, and a sense of oppression. It is a re- 

 markable fact, however, that the organism after a time adapts itself to 

 a very vitiated atmosphere, and that a person soon comes to breathe, 

 without sensible inconvenience, an atmosphere which, when he first en- 

 ters it, feels intolerable. 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 2,000 cubic feet per head should be allowed in sleeping apart- 

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

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

 tilation as provides for the supply of 1,500 to 2,000 cubic feet of fresh 

 air per head per hour. (Parkes.) 



The Effect of Respiration on the Circulation. 



As the heart, the aorta, and pulmonary vessels are situated in the 

 air-tight thorax, they are exposed to a certain alteration of pressure 

 when the capacity of the latter is increased in inspiration; for although 

 the expansion of the lungs tends to counter-balance this increase of area, 



