124 RESPIRATION 



is far more common than was, till recently, supposed. Portions of 

 the skin may be blue from local slowing of the circulation due to 

 cold and other causes; but abnormal blueness of the lips and 

 tongue points to either imperfect oxygenation of the arterial 

 blood or general slowing of circulation. According as there is 

 much or little blood in the capillaries the color is full or unsatu- 

 rated. Thus in extreme cyanosis the lips may be either almost 

 black, or only leaden gray ; and in slight cyanosis the color may 

 be either a full or a pale purplish red. 



Ordinary cyanosis of one kind or another is commonly seen in 

 patients who, though suffering from some chronic ailment, are 

 not particularly ill. Hence the significance of cyanosis under other 

 conditions is apt to be overlooked unless all the symptoms and 

 other circumstances are taken into account. It must, in the first 

 place, be pointed out that the degree of cyanosis is no direct 

 measure of the degree of physiological anoxaemia. The latter is 

 due to a lowering in the partial pressure of oxygen in the blood of 

 the capillaries, while the former is due to a diminution in the ratio 

 of oxyhaemoglobin to haemoglobin. Under ordinary conditions 

 the latter effect is an index, though, owing to the form of the 

 dissociation curve of oxyhaemoglobin (Figure 20), not a direct 

 measure, of the former effect. When, however, the matter is com- 

 plicated by an alteration in the Bohr effect of CO 2 pressure on the 

 dissociation of oxyhaemoglobin, the relationship between oxygen 

 pressure and dissociation of oxyhaemoglobin is at once altered. If, 

 for instance, the pressure of CO 2 in the arterial blood is reduced 

 by increased breathing, there may be much less cyanosis for a 

 given degree of physiological anoxaemia than when the CO 2 

 pressure in the blood is normal. Thus there is no fixed relationship 

 between cyanosis and physiological anoxaemia; and this fact is 

 of great importance in the clinical interpretation of cyanosis. 

 Moreover, as Barcroft showed, the Bohr effect is due to the action 

 of CO 2 as an acid. Hence, owing to the adjustments which, as will 

 be shown in Chapter IX, occur in the living body when time is 

 given, the CO 2 pressure in the alveolar air may be no guide as to 

 how far the Bohr effect is disturbing the ordinary relations be- 

 tween cyanosis and true anoxaemia. The word "anoxaemia" 

 should evidently be taken as signifying a condition in which the 

 free oxygen in the systemic capillary blood is abnormally dimin- 

 ished ; and this of course, in accordance with Henry's law, comes 

 to the same thing as diminution in the oxygen pressure. 



The symptoms produced in the nervous system generally by 



