106 RESPIRATORY MECHANISMS 



the mode of respiration. His results are illustrated in Fig. 61. 

 Between the two curved lines the respiration was mixed, 

 while outside them it was purely aerial and inside purely 

 aquatic. The most interesting point is that aerial respiration 

 is resorted to at low CO2 tensions, even when the oxygen 

 content of the water is high. This appears to show that C0 2 

 at tensions from about 10 to about 25 mm is necessary as a 

 stimulus to branchial respiration, an adaptation which, is 

 probably unique among fishes and foreshadows regulation 

 mechanisms in much higher forms. Adaptation to oeco- 

 logical conditions is evidently specific and of very little value 

 as a character of larger systematic groups. 



Hemoglobin as an oxygen store. In all cases where haemo- 

 globin is present in tissue cells and does not circulate at all the 

 only function which can at present be assigned to it is to act as 

 a store of oxygen on which the tissue can draw when the sup- 

 ply from the blood fails or is inadequate. This will usually 

 involve the assumption that the tissue haemoglobin is normally 

 saturated with oxygen and shows a steeper dissociation curve 

 than the haemoglobin of the blood. The studies of H. Theorell 

 (1934) have shown this to be the case. The t u of horse-muscle 

 haemoglobin is 3.6 mm, and Theorell points out further that 

 the effect of C0 2 or acidity is very slight and that this may have 

 something to do with the fact that in muscles it may be exposed 

 to larger variations than in the blood. This reasoning is valid 

 only when we assume that the time factor is important in the 

 utilization of the oxygen store. The distribution of myoglobin 

 is curious. It seems always to be present in the heart. It is 

 generally present in skeletal muscles in higher vertebrates, 

 but absent in fishes (one exception noted by Ray Lankester is 

 the muscles of the dorsal fin of Hippocampus). The concen- 

 tration varies greatly. As mentioned on p. 81 it is very high 

 in diving mammals and probably also at high altitudes (p. 102). 

 It appears to increase with age, as indicated by the difference 

 in colour between veal and beef. It is generally higher in 

 wild animals than in tame of the same species, but it is nearly 



