648 PHYSIOLOGY OF RESPIRATION. 



tree, sometimes known as the "dead space" of the lungs, since 

 the air filhng this space is not useful in the respiratory processes. 

 The average size of the "dead space" is of importance in many of 

 the calculations in regard to the extent of respiratory exchange, but 

 the figures reported by different experimenters vary considerably. 

 According to observations based mainly on anatomical measure- 

 ments* the volume of the dead space has been reckoned usually as 

 equal to about 140 c.c. for the adult. Some recent observers mak- 

 ing use of an indirect method of calculation based upon the con- 

 centration of the CO2 in the expired air and the alveolar air have 

 stated that the size of the dead space may vary with conditions. 

 In the dyspnea or hyperpnea due to muscular exercise it may 

 be increased to as much as 600+ c.c. (Douglas and Haldane) on 

 account of a dilatation of the bronchioles or atria. Others, while 

 admitting some variation with differences in the extent of the 

 respiratory movements, state that the variation is relatively 

 small, lying between limits of 100 to 200 c.c.f If we accept the 

 figures, 140 c.c, as representing a mean value, then at each inspira- 

 tion at least 360 c.c. of air penetrate into the alveoH, and if evenly 

 disseminated through the lungs add about -jV ^^ ^^^^ volume of 

 each alveolus. Once in the alveoli, diffusion must tend to spread 

 the tidal air rapidly, and that this occurs is shown by an interesting 

 experiment performed by Grehant. He breathed in 500 c.c. of 

 hydrogen instead of air and 'then examined the amounts of hy- 

 drogen breathed out in successive expirations. Only 170 c.c. 

 were recovered in the first expiration, 180 c.c. in the second, 41 in 

 the third, and 40 in the fourth. 



Artificial Respiration. — In laboratory experiments artificial 

 respiration is employed frequently after the use of curare ; when it is 

 necessary to open the chest: after cessation of respirations from 

 overdoses of chloroform or ether, etc. The method used in almost 

 all cases is the reverse of the normal procedure, — that is, the lungs 

 are expanded by positive pressure (pressure in excess of atmos- 

 pheric). A bellows or blast worked by hand or machinery is con- 

 nected with the trachea and the lungs are dilated by rhythmical 

 strokes. Provision is made for the escape of expired air by the use 

 of valves or by a side hole in the tracheal cannula. Numerous 

 forms of respiration pumps have been devised for this purpose. 



In cases of suspended respiration in human beings from drown- 

 ing, electrical shocks, pressure upon the medulla, etc., it is necessary 

 to use artificial respiration in order to restore normal breathing. 

 Bellows ordinarily cannot be used in such cases. Some method 



* See Loewy, "Archiv f. die gesammte Physiologie," 58, 416. 

 t Krogh and Lindhard, "Journal of Physiology," 41, 59, 1917; also Pearce, 

 "American Journal of Physiology," 43, 73, 1917. 



