134 RESPIRATION. 



undoubtedly so regard it, if it were not for the fact that 

 we require it constantly, instead of taking it at stated in- 

 tervals, as is the case with our articles of diet. Again, as 

 the demand of the system for food is expressed by the sen- 

 sation of hunger, so the demand for air is marked by a 

 painful sensation called suffocation. 



21. Interchange of Gases in the Lungs. As the 

 air and the blood are not in contact, they being separated 

 from each other by the walls of the air-cells and of the 

 blood-vessels, how can the two gases, oxygen and carbonic 

 acid, exchange places ? Moist animal membranes have a 

 property which enables them to transmit gases through 

 their substance, although they are impervious to liquids. 

 This may be beautifully shown by suspending a bladder 

 containing dark blood in a jar of oxygen. At the end of 

 a few hours the oxygen will have disappeared, the blood 

 will be brighter in color, and carbonic acid will be found 

 in the jar. 



22. If this interchange takes place outside of the body, 

 how much more perfectly must it take place within, where 

 it is favored by many additional circumstances! The 

 walls of the vessels and the air-cells offer no obstacle to 

 this process, which is known as gaseous diffusion. Both 

 parts of the process are alike of vital importance. If oxy- 

 gen be not received, the organs cease to act; and if car- 

 bonic acid be retained in the blood, its action is that of a 

 poison ; unconsciousness, convulsions, and death follow- 

 ing. 



23. Difference between Arterial and Venous 

 Blood. The following table presents the essential points 

 of difference in the appearance and composition of the 

 blood, before and after its passage through the lungs : 



21 . Moist animal membranes ? How shown with the bladder ? 



22. Gaseous diffusion? If oxygen be not received? If carbonic acid be re- 

 tained? 



23. Difference in the appearance and composition of the blood? Temperature 

 of the hlood ? The blood while passing through the lungs ? The consequence ? 



