THE CHEMISTRY OF RESPIRATION 



1113 



wall will remain the same, so that the consumption of oxygen by the tissues 

 will be unaltered by changes of the oxygen tension in the blood. Tested 

 by this method Verzar found that the submaxillary gland falls within the 

 second case, and that its oxygen tension must be but little removed from 

 that of the blood itself. Altering the oxygen tension of the blood by making 

 an animal breathe a mixture poor in oxygen does not affect in any way 

 the oxygen consumption in the gland. On the other hand, in resting muscle 

 Verzar found that the oxygen tension is probably zero, so that its consump- 

 tion could be widely affected by diminishing the oxygen tension in the air 

 supply to the animal. Krogh has attacked the question in a different 

 manner. In a series of experiments he first determined the rate of diffusion 

 of oxygen through different thicknesses of various tissues (connective tissue, 

 muscle, etc.) under varying pressures. Then, partly on the living muscle, 

 -partly on injected specimens, he estimated the relation of the number and 

 size of the capillaries to the intercapillary muscular tissues. He found the 

 pressure differences necessary to supply the muscle fibres with oxygen 

 extremely email. Some of his results on the guinea-pig are shown in the 

 accompanying JTable . 



The first thing that strikes us in this Table is the enormous difference 

 between the capillary circulation of resting and that of active muscle. ' In 

 the resting muscle the majority of the capillaries are empty and collapsed, 

 so that large areas of muscle intervene between the few capillaries in which 

 the circulation of blood is proceeding. Under these conditions the pressure 

 difference necessary to supply the total oxygen consumed by the muscle, 

 e. g. 45 mm. Hg., may fall below the venous oxygen tension, so that in parts 

 of the muscle the oxygen tension may be zero, as maintained by Verzar. 

 After massage a number of capillaries open, and the number is still further 

 increased by work, so that there may be a hundredfold increase in the 

 number of capillaries in every square millimetre of a cross-section of the 

 muscle. Under these conditions the passage of oxygen from the capillaries 

 is so facilitated that the oxygen pressure in the muscle tissues becomes 

 practically equal to that of the blood. It would appear that, so far as the 

 supply of oxygen to the muscle is concerned, the increase in the capillary 

 area during muscular exercise is far ahead of the actual needs of the muscle. 



