RESPIRATION 635 



consciousness, sudden, unexpected, and irresistible." When Tissandier recovered 

 consciousness, both of his companions were dead. Although all were provided 

 with oxygen to inhale, they were paralysed before they were aware of the fact, 

 and therefore unable to take hold of the tubes. 



SUMMARY 



The object of the respiratory mechanism is to provide for a supply of oxygen to 

 the tissues of the larger animals, where direct access is impossible, and for the 

 escape of the carbon dioxide formed, in combustion. 



The oxygen has to be taken up from the air and conveyed to the tissues in 

 solution in a liquid, the blood. In the tracheate bisects, the gas is supplied 

 directly to the tissues by means of fine tubes containing air. 



The necessity of continued supply of fresh air was proved by Robert Hooke in 

 1667, and the essential constituent, oxygen, was discovered by John Mayow in 

 1674. Black, in 1755, showed that the product of combustion in animals 

 is different from common air and called it " fixed air " ; its nature as an oxide of 

 carbon was discovered by Lavoisier in 1775. 



There is no evidence that oxygen is stored in the cells in an " intra-molecular " 

 or other form available for oxidation, with the exception of the very minute amount 

 present as peroxide. Such conceptions as that of " biogen molecules " are not in 

 agreement with experimental facts. 



The phenomena of narcosis are not due to inhibition of oxidation, but to changes 

 in the properties of the cell membrane. 



Consideration of " life without oxygen " leads to the view that the actual source 

 of the free energy required by a living organism is a secondary matter. If it 

 cannot be obtained by oxidation, other chemical reactions, although of a less 

 efficient kind, are made use of. 



Data of the actual consumption of oxygen by various tissues are given in the 

 text. The heart is found to use oxygen in direct proportion to the tension energy 

 developed. There is no more consumption of oxygen by the lungs themselves than 

 by other organs in rest ; that is, there is no evidence of oxidation of metabolic 

 products from other organs and contained in the blood. The actual amount of 

 oxygen consumed in the lung tissue is only about half that consumed by the 

 salivary gland at rest. The blood itself in mammals only consumes minimal 

 amounts of oxygen, except when large numbers of young cells are present, as after 

 anaemia. Nucleated red blood corpuscles consume considerable amounts of oxygen. 



Since the supply 'of oxygen required is much greater than could be carried in 

 ordinary solution, there is a special substance, haemoglobin, in the red corpuscles, 

 which has the remarkable property of taking up oxygen in amount proportional to 

 the pressure of the gas. Thus it takes up oxygen in the lungs and passes it on to 

 the tissues where the oxygen tension is low. 



Haemoglobin contains iron, and it is held by many that the oxygen taken up is, 

 in some way, in combination with the iron, since it appears to be in molecular 

 proportion to it. 



There is no chemical system known which has properties like those of the 

 oxygen-haemoglobin system. Hence it seems probable that surface phenomena act 

 as controlling factors in the amount of the " compound " present at a given oxygen 

 tension. But no satisfactory explanation has been as yet suggested. 



The amount of oxygen which haemoglobin can take up at a given tension is 

 lowered by rise of temperature and by the presence of neutral salts or acid. The 

 importance of these facts with regard to its function is pointed out in the text. 



A brief account of the phase rule is given in the text, in connection with the 

 possibility of its application to the case of hemoglobin. 



It is found that an exponential formula expresses the relation of haemoglobin 



