I. — PHYSIOLOGY. 163 



3uced by the American researcher Stadie,* is the method of arterial 

 puncture. It had long been the wish of physiologists to make direct 

 examinations of the gases in human arterial blood, yet, as far as I know, 

 this had only once been accomplished, namely, by Dr. Arthur Cooke 

 and myself in a case in which the radial artery was opened for the pur- 

 pose of transfusion. But the matter now seems to be relatively 

 simple. The. needle of a Hypodermic syringe can be put right into the 

 radial artery and art-erial blood withdrawn. I am not sure that the 

 operation is less painful than that of dissecting out the radial artery 

 and opening it — and in this matter I speak with experience — but it is 

 less alarming, and it has the great merit that it does not injure the 

 artery. 



Anotlier method of determining the percentage saturation of arterial 

 Blood has invited the attention of researchers, appearing like a will- 

 o'-the-wisp, at one time within grasp, at another far off. That method 

 is to deduce the percentage saturation from the composition of the 

 alveolar air. Into the merits of the rival methods for the determina- 

 tion of the oxygen in alveolar air I will not go : the method of Haldane 

 and Priestley will suffice for persons at rest. Granted, then, that a 

 subject has a partial pressiure of 50 mm. of oxygen in his alveolar air, 

 what can we infer as regards his arterial blood ? A long controversy has 

 raged about whether or no any assumption could be made about the 

 condition of the arterial blood from that of the alveolar air, for it was 

 an article of faith with the school of physiologists which was led by 

 Haldane that when the oxygen pressure in the alveolar air sank, tlie 

 oxygen in the arterial blood did not suffer a corresponding reduction. 

 The experimental evidence at present points in the opposite direction, 

 and unless some further facts are brought to light it may be assumed 

 that the oxygen pressure in the arterial blood of a normal person at 

 rest is some five millimetres below that in his alveolar air. And having 

 obtained a figure for the pressure of oxygen in the arterial blood, where 

 do we stand as regards the percentage of saturation? The relation 

 between the one and the other is known as the oxygen dissociation 

 curve. It differs but slightly in normal individuals, and at different 

 times in the same individual. To infer the percentage saturation from 

 the oxygen pressure, no doubt the actual dissociation curve should be 

 determined, but in practice it is doubtful whether as a first approxi- 

 mation this is necessary, for a curve determined as the result of a few 

 observations is unlikely to be much nearer the mark than a standard 

 curve on which twenty or thirty points have been determined. 

 Therefore an approximation can be made for the percentage saturation 

 as follows : In a normal individual take the oxygen in the alveolar air, 

 subtract five millimetres, and lay the result off on the mean dissociation 

 curve for man. 



Whether measured directly or indirectly, the answer is a statement 

 of the relative quantities of oxyhfemoglobin and of reduced haemoglobin 

 in the arterial blood. The important thing is that there should be as 

 little reduced haemoglobin as possible. The more reduced haemoglobin 

 there is present the less saturated is the blood, or, as the American 

 authors say, the more imsaturated is the blood. They emphasise 



H 2 



