638 Journal of Agriculture. [10 Oct., 1908. 



only hold about half a volume of oxygen, but if the corpuscles are present, 

 about 20 volumes of oxygen. Moreover, a solution of haemoglobin 

 will act in the same way. One very important character of 

 haemoglobin is that, if the oxvgen of the air, with which it is in contact, 

 be lowered from the normal 20.9 per cent, to 13 per cent., the deficit in 

 the oxygen absorbed by the haemoglobih is trifling; but if the oxygen be 

 still further lowered in a series of equal gradations, then the deficit 

 becomes larger and larger until small differences in the oxygen of the air 

 mean large differences in the amount absorbed. There is nothing very 

 remarkable in this behaviour of haemoglobin. One can see much the same 

 phenomenon in the ab.sorption of carbon-dioxide by a solution of ordinary 

 washing soda. But to the animal it has a profound significance. It 

 means that the oxygen content of the air can be lowered (either by high 

 altitudes or by dilution with nitrogen) and yet, within certain bmits of 

 course, the amount of oxygen ab.sorbed by the blood is little altered. Thus 

 an animal can live in an atmosphere in which a candle cannot burn, pro- 

 vided that poisonous gases are not present ; and mountaineers frequently 

 attain to-day to heights of 18,000 feet where the mass of oxygen per 

 unit volume of air is reduced to one-half that at sea-level. 



In the lungs the blood does not come into immediate contact with the 

 air, but the oxygen of the latter can readily diffuse through the thin lining 

 of the air-cells and through the capillary wall and into the bl<X)d. Through 

 the same membranes the carbon-dioxide of the blood can diffuse outwards. 

 The carbon-dioxide of the blood is held partly in .simple solution, partly 

 in the form of bicarbonates. It is also probable that the haemoglobin can 

 help in carrying some, but a small, portion. 



The nitrogen in the blood is simply dissolved and plavs no part in 

 the animal economv. Mention must be made here of the poisonous action 

 of carbon-monoxide, a gas which is produced bv glowing charcoal and 

 incomplete combustions such as occur when a flame plays on a metal 

 surface, or when dust is roasted by contact with hot metal, &c. This gas 

 has an extraordinary affinitv for haemoglobin and in concentrations of i 

 volume per 1,000 volumes of air can oust the greater part of the oxygen 

 from the haemoglobin. Carbon-monoxide injures or kills not by any 

 specific poisonous action but simplv because of oxygen star\ation. 



INTERNAL RESPIRATION. 



Every living cell in the body needs oxygen for combustion purposes 

 either to furnish energy or to remove organic waste. When arterial blood 

 reaches the thin-walled capillaries its oxygen can readily diffuse out into 

 the lymph and .so into the living cells where oxidation takes place. Now 

 the chief products of cellular oxidation are water and carbon-dioxide both 

 of which diffuse out of the cell into the blood. Hence arterial blood 

 passing through the capillaries becomes venous, that is, loses some oxvgen 

 and gains almost as much carlx)n-dioxide. The more active tTie cell, the 

 more oxygen does it require and the more carbon- dioxide does it pour out. 



RESPIRATORY EXCHANGE. 



The amount of oxygen absorbed, and conversely, of carbon-dioxide gi\en 

 out, varies within wide limits according to the size of the animal and to the 



