166 Chapter X 



percentage saturation is arrived at. We can, then, determine the 

 percentage saturation of the arterial and venous bloods and, by 

 laying them off on the dissociation curve, we can determine the 

 pressure of oxygen in the artery and the vein, and in that way at least 

 ascertain within limits the pressure in the capillaiy at a given time. 

 We need scarcely interrupt the course of our discussion to point out 

 to the reader the extreme fallibility of the methods in which we are 

 engaged. We are in the position of one navigating a difficult channel 

 in foggy weather. Nevertheless, it may be that the points which we 

 have to observe are sufficiently obvious to stand out even in the fog, 

 that in short there are fixed laws determining the pressure in the 

 tissues which can be appreciated by methods as fallible as the best at 

 present within our reach. 



Let us assume then that we can determine Q and p. Can we 

 now vary one or other of these and see whether the relation holds 

 Q cc p ? By the following means Verzar varied p. The cat which was 

 anaesthetised with urethane was made to breathe gas from a gaso- 

 meter by means of water valves attached to the trachea tube. The 

 gasometer contained air, but as the animal drew upon the supply 

 of air, the gasometer in turn drew upon the contents of another much 

 larger gasometer which was filled with nitrogen, this in turn being 

 displaced by water. Thus the cat was getting gas which constantly 

 and gradually became less and less rich in oxygen, and in time the 

 oxygen pressure in its arterial blood began to fall visibly as indicated 

 by the colour of the blood. When this process had gone far enough, 

 samples of blood were again taken for analysis. 



For the following reasons it will be best to consider the experiments 

 made on the salivary gland first. They gave very uniform results 

 which were easy to interpret. They were free from the complica- 

 tions of alteration in the blood flow since it has been shown by Miiller 

 and myself 12 ' that the amount of oxygen acquired from the blood by 

 the resting submaxillary is not appreciably affected even by great 

 changes in the rate of flow. In the third place experiments on the 

 submaxillary follow naturally upon the investigations described 

 above relating to intra-capillary pressure of oxygen in the same 

 organ. What a satisfactory organ the submaxillary gland is easy 

 of access, easy of control, relatively unaffected by the usual 

 anaesthetics ! 



The experiments can best be understood by following a single 

 one in detail. Let it be Experiment I. 



