FUNCTIONAL ADAPTATION — SUBMAMMALIAN PIGMENTS 331 



0.5%, and under normal conditions of activity the oxygen store 

 would last for about seven seconds. In extreme activity the store 

 would be used up in each contraction. Millikan's experiments (1953) 

 have indeed shown that in this species, and probably in other terrestial 

 mammals, the storage function of myohemoglobin is related to the 

 frequency with which a muscle is able to contract. The dissociation 

 curve of myohemoglobin is admirably adapted to its function. At 

 the oxygen tension of venous blood, myohemoglobin is 94% saturated 

 (1279). The apparent loading tension of the respiratory ferment is 

 low enough for it to operate at the pressure required to dissociate 

 about 50% of the oxygen bound to myohemoglobin. 



Actual measurement of the rate of reduction of myohemoglobin 

 during tetanic contraction of the soleus muscle of the cat showed 

 that the velocity of dissociation was of the same order as that of the 

 increase of muscular tension during contraction. Oxygen consump- 

 tion commenced less than 0.2 second (instrumental lag) after the 

 application of the stimulus. Since within one second the myooxy- 

 hemoglobin lost up to 40% of its oxygen, in spite of the fact that the 

 circulation was left intact, the oxygen supply to the muscle is insuffi- 

 cient during the contraction. Millikan envisages the storage function 

 of myohemoglobin as being able to provide a supply of oxygen when 

 the need is greatest, as well as smoothing out the fluctuations in oxygen 

 content during intermittent action. 



In diving mammals, the myohemoglobin content of the muscles is 

 particularly high (65^,2288,2763) and in addition to the above role it 

 probably enables the animals to stay under water for long periods. 

 In the dolphin and seal, 3.5 and 7.7% are found, respectively, amounts 

 which roughly correlate with the duration of their dives. 



11. FUNCTIONAL ADAPTATION — SUBMAMMALIAN 

 PIGMENTS 



11.1. Diversity of Environments 



In the two previous sections, the chemical bases of the adaptation 

 of hemoglobin and myohemoglobin to their histological environment 

 have been discussed. In the case of the submammalian oxygen- 

 binding pigments, we are hampered not only by knowing less about 

 the pigment, but also by knowing less about the functional role 

 which the pigment plays in the particular species. Certain aspects 



