RESPIRATION 245 



1 fixed ' in some way or other. The traces of oxygen in the 

 lymph cannot therefore be journeying away from the tissue 

 elements ; they must have come from another source, and 

 this can only be the blood. Could we gather lymph for 

 analysis directly from the thin sheets that lie between the 

 blood capillaries and the tissues, we might find more oxygen 

 present as well as more carbon dioxide. But if we did find 

 more oxygen, it would still be oxygen in transit from the 

 capillaries towards places where the partial pressure of 

 oxygen is less. In the lymph, the pressure is kept low by 

 the avidity of the tissues with which it is in contact, and 

 possibly by the existence in it of oxidizable substances which 

 have come from the tissues. In the tissues there is no 

 partial pressure at all, because the oxygen that reaches 

 them is at once stowed away in some compound, in which 

 it has lost the properties of free oxygen. 



Assuming, then, that at least a great part of the oxidation 

 and consequent production of carbon dioxide goes on in the 

 tissues, let us follow the steps of the process, as far as we can, 

 in the light of our knowledge of the respiration of muscle. 



Respiration of Muscle. Three methods have been used to 

 determine the respiratory changes going on in resting muscle, 

 or to compare them with those in the excited state : 



(1) The excised muscles of cold-blooded animals are exposed for 

 a considerable time to an atmosphere of known composition in a 

 small chamber ; and the changes in this atmosphere are then deter- 

 mined. Or, what is better, especially for short periods of observa- 

 tion, a stream of air or nitrogen is drawn through the chamber and 

 the carbon dioxide absorbed by baryta water, which is then titrated 

 with standard acid. 



(2) Samples of the blood coming to and leaving a muscle of a 

 warm-blooded animal may be taken in its natural position, and the 

 gases analyzed and compared. 



(3) Artificial circulation may be kept up through a muscle or 

 group of muscles ; for example, through one or both hind-limbs of a 

 dog. The blood may be oxygenated in a special chamber from a 

 graduated cylinder containing oxygen, and the carbon dioxide col- 

 lected in baryta or caustic potash valves. The oxygen consumption 

 can be read off from the cylinder, and the production of carbon 

 dioxide estimated by titrating the baryta water or potash (Fig. 85). 



By the first of these methods a very remarkable fact, 

 among others, has been brought to light. It has been 



