APPARATUS FOR PHYSIOLOGICAL USE. 261 



escaping by the tube h. The stop-cock y is then closed, and b 

 being again lowered to the position of b' the mercury flows 

 back into it, leaving a vacuum in a. The blood is intro- 

 duced into the receiver u by allowing it to flow in directly 

 from a blood-vessel of an animal. It is then attached to 

 the air-pump, but shut off from any communication with 

 it by the stopcock t. The whole of the rest of the apparatus 

 being then exhausted by raising and lowering b in the 

 manner already described, or, as we may term it for con- 

 venience sake, by one or two strokes of the pump, the stop- 

 cock t is opened, and the gases of the blood rush into a. 

 In their passage over the sulphuric acid in c they are dried, 

 and by again raising b they are driven through h into a 

 tube filled with mercury and placed over the pneumatic 

 trough, but not represented in the diagram. A few more 

 strokes of the pump extract all the gases, drive them into 

 the tube just mentioned, and there they are analysed in 

 the ordinary way, which it is unnecessary to describe here. 



I mentioned to you at the beginning of this lecture that 

 the muscle of a frog gives off carbonic acid during contrac- 

 tion, and that this evolution of carbonic acid can be 

 ascertained easily and directly by simply putting the muscle 

 in a tube and analysing the gases which the tube contains 

 after the muscle has been some time in it. But you cannot 

 find out by this simple and direct process that the muscle 

 uses up oxygen during its contraction, for it does not take 

 up oxygen to any extent from the air which surrounds it, 

 but abstracts it from the blood which circulates through 

 it, and stores it up within itself, so that it can continue to 

 contract even after the circulation has been arrested. 

 While the excretion of carbonic acid then can be examined 

 directly, the absorption of oxygen by living muscle can 

 only be investigated indirectly by examining the composi- 

 tion of the gases of the blood before and after it has passed 

 through the muscle, and ascertaining whether the oxygen 

 has diminished during the passage or not. On doing 

 this it is found that the oxygen is diminished while the 

 carbonic acid is increased, and by this means it has been 

 finally settled that oxygen is used up and combustion goes 

 on in the tissues, such as muscle, themselves, and not in 

 the lungs or blood only. 



We have next to ascertain how much oxygen is used up 



