RESPIRATION. 



205 



a quantity of air in which either, separately, would survive three 

 >urs, they will not live 1 hour, but only 1 hour. 



From what has been said it must be evident that provision for a con- 

 it and plentiful supply of fresh air, and the removal of that which is 



itiaU'd, is of far greater importance than the actual cubic space per head 

 occupants. Not less than 2000 cubic feet per head should be allowed 

 sleeping apartments (barracks, hospitals, etc.), and with this allow- 

 the air can only be maintained at the proper standard of purity by 



ich a system of ventilation as provides for the supply of 1500 to 2000 



ibic feet of fresh air per head per hour. (Parkes.) 



THE EFFECT OF RESPIRATION ON THE CIRCULATION. 



Inasmuch as the heart and great vessels are situated in the air-tight 

 lorax, they are exposed to a certain alteration of pressure when the 



FIG. 162. Diagram of an apparatus illustrating the effect of inspiration upon the heart and great 

 vessels within the thorax. I, the thorax at rest; If, during inspiration: p, represents the diaphragm 

 when relaxed; D' when contracted (it must be remembered that this position is a mere diagram), i. e., 

 when the capacity of the thorax is enlarged; H, the heart; v, the veins entering it, and A, the aorta; 

 R/. Ll, the right and left lung; T, the trachea; M, mercurial manometer in connection with the pleura. 

 The increase in the capacity of the box representing the thorax is seen to dilate the heart as well as 

 the lungs, and so to pump in blood through v, whereas the valve prevents reflex through A. The 

 position of the mercury in M shows also the suction which is taking place. (Landois.) 



capacity of the latter is increased; for although the expansion of the 

 lungs during inspiration tends to counterbalance this increase of area, it 

 never quite does so, since part of the pressure of the air which is drawn 



