138 KSSKNTIAI.S OF CMIKMICAL PHYSIOLOGY 



more, carbonic acid ; thus, in the case of a dog, when the oxygen 

 tension in the blood is approximately 18 millimetres of mercury or 

 one fortieth of an atmosphere, 163 c.c. of oxygen per minute were 

 used up by the animal, and 172 c.c. of carbonic acid were given out ; 

 normally by the same animal the oxygen consumption was 157 c.c. 

 and the carbonic acid output 158 c.c. The reason why the tissues 

 extract oxygen with such readiness from the blood, even when the 

 oxygen exists in the blood at a low tension, is that there is no free 

 oxygen in the tissues themselves (and they always thirst for it). This 

 fact can be demonstrated in more than one way. 



(1) No oxygen can be extracted from the tissues by exposing them 

 to the vacuum of an air-pump. 



(2) The tissues possess the power of reducing such substances as 

 methylene blue (see p. 128). 



Respiration in excised Tissues, Excised frog's muscle retains its 

 power of contraction for a considerable time. During this time it 

 gives out carbonic acid. These facts are true whether the muscle be 

 in air or in nitrogen. In either case the muscle must be regarded as 

 partially or entirely asphyxiated, for the individual elements of the 

 muscle are cut off from that ready supply of oxygen which normally 

 reaches them. During life (and the living condition can be imitated 

 by placing an excised muscle in an atmosphere of pure oxygen) the 

 muscular substance breaks down into a number of somewhat simpler 

 substances : one of these is carbonic acid. The others, however, or 

 some of them, are at once built up again with the inclusion of oxygen 

 and some carbon-containing substance, perhaps sugar, into living 

 material. The muscle, therefore, does not contain any of the by- 

 products of its own metabolism. In excised muscle, when the oxygen 

 supply is deficient the by-products accumulate, as a result of which 

 very striking alterations take place. (1) The reaction of the muscle 

 becomes acid and the phenomena of fatigue and functional death set 

 in. (2) The proteins become coagulated, and this is the physical basis 

 of rigor mortis. 



The Chemical Stimulus to Respiration. Haldane and Priestley 

 have introduced a new and simple method of obtaining the com- 

 position of the air in the alveoli. It consists in collecting one 

 sample of air expelled by a deep expiration at the end of a quiet 

 inspiration, and another of the air expelled by a deep expiration at 

 the end of a quiet expiration ; the mean of the two gives the com- 

 position of alveolar air. This is much simpler than the method 

 formerly employed by Pfliiger, which consisted in pumping off the 

 air from an occluded portion of a dog's lung by the means of a fine 



