1883.] THE VENTILATION OF FARM BUILDINGS. 195 



large bronchial tube or wind -pipe; with its divisions and sub-di- 

 visions, which terminate in minute branches, which are surrounded 

 by air-cells. In the mammalia it is estimated that there are 

 about 20,000 air cells clustered around each terminal branch, and 

 that the total number in the human species is not far from six hun- 

 dred millions, presenting a surface of about 160 square yards. 

 Now consider that the walls of these cells are closely packed with 

 fine capillary vessels, which are terminations of arterial sub-divi- 

 sions, and we can see how the blood of the system can be exposed 

 to the air, for all the blood of the body must pass through the 

 lungs in very short periods, varying in our different animals ac- 

 cording to the frequency of the heart's action. The number and 

 minuteness of the capillaries, and the vast expanse of lung surface 

 accomplishes its aeration. 



By respiration we see the following objects. It establishes the 

 circulation of the blood at the commencement of life, the functional 

 activity of all muscular and nervous tissues depend upon their 

 oxydation, and this can only be by the introduction of air; the 

 maintenance of heat is mostly due to the oxydation of disinte- 

 grated material of the body that is passing to waste, and of other 

 combustibles, such as fat and sugar. All organic material at its 

 death gives rise to two products under the action of the air, car- 

 bonic acid gas, and water. * The condition of life being such as 

 not to allow of the accumulation of this gas in the system, means 

 for its removal have been established; the introduction of oxygen, 

 and the removal of carbonic acid are accomplished by the same 

 mechanism. 



You may naturally ask the question, how does the actual oxygen 

 of the air get into the blood, for it is confined in small capillaries; 

 the wall of the vessel is between the blood and the air? It is 

 explained partly by the law of the diffusion of gasee, and partly 

 by another law of capillary attraction, or, as it is called, endos- 

 mosis, and exosmosis, which will explain how gases and fluids 

 can pass through membranes, liquid films, or porous structures. 

 The whole thickness of a capillary vessel is only about -g-^Vo °^ ^^ 

 inch ; therefore its walls must be extremely thin and present but 

 very little obstruction to the passage of a gas. 



"When the oxygen is taken into the blood in the lungs, what 

 then becomes of it ? To answer this question we must bring the 



