RESPIRATION 259 



results to man, and interpreted them in the light of the thin line 

 in the dissociation curves of oxyhaemoglobin shown in Figure 28 

 (assuming that the haemoglobin of arterial blood is 95 per cent 

 saturated) and the thick line in the corresponding curve for CO 2 

 (Figure 26) it would appear that the average pressure of oxygen 

 in the venous blood is about 5.2 per cent of an atmosphere, or 

 40 mm. of mercury, and the average pressure of CO 2 about 47 mm. 

 The experiments were, however, made on animals, while the dis- 

 sociation curves (the only accurately determined ones) are for 

 human blood. Moreover the animals, owing to operative disturb- 

 ances, anaesthetics, etc., were more or less under abnormal con- 

 ditions. Hence the inferences -just drawn are mere approxima- 

 tions. The very great variability in the CO 2 content of the samples 

 of arterial blood from animals of the same species, as compared 

 with the constancy of CO 2 content in the case of man under normal 

 resting conditions, is in itself very significant. The history of the 

 investigations detailed in the preceding chapters is sufficient to 

 warn us of the necessity for reaching more than rough approxima- 

 tions in physiological investigation, and for expecting that physio- 

 logical regulation of the circulation may turn out to be something 

 just as delicate and definite as regulation of respiration. It is to 

 measurements in man, rather than in animals, that we must look 

 for information of sufficient physiological accuracy, just as it has 

 been through measurements in man that our definite information 

 as to the regulation of breathing has been obtained. 



The difficulty as regards human experiments has till quite 

 recently been that of suitable methods. We can easily measure 

 the blood pressure, pulse rate, etc., in man; but the information 

 thus obtained is extremely limited in value and almost impossible 

 to interpret satisfactorily in the absence of information as to the 

 rate of blood flow. Direct measurements of the rate of blood flow 

 in animals have been carried out by means of the Ludwig 

 "Strohmuhr" and the improved forms of it which have been 

 applied to measuring the blood flow through the aorta; but the 

 operative disturbance is far too serious to allow of sufficiently 

 definite results being obtained. Valuable information of a rough 

 kind was obtained by Zuntz and Hagemann 1 in experiments in 

 which the gases of the venous and arterial blood were determined 

 in horses, along with the total respiratory exchange, during rest 

 and work. These experiments seemed to show clearly that the 



1 Zuntz and Hagemann, LancLwirtsch. Jahrb., 27, Supplem. Bd. Ill, 1898. 



