THE TISSUES 



69 



2. Respiration of Excised Muscle. By enclosing the ex- 

 cised muscle in a closed space containing air of known com- 

 position, and by investigating the changes in the components 

 of the air after the muscle has either been kept at rest for 

 some time or made to contract, important light has also been 

 thrown on these chemical changes. 



It has been found that the resting muscle constantly takes 

 up oxygen from the air round about it, and constantly gives 

 off carbon dioxide. In contracting, more carbon dioxide is 

 given off, and usually the amount of oxygen taken up is also 

 increased. (Fig. 30.) 



Here we have at once evidence that muscle breathes, and 

 that this process of respiration is increased during muscular 



FIG. 30. Respiration of muscle in a closed chamber. 



activity. The affinity of muscle for oxygen is very great, so 

 great that jt can actually talrp nvrrpn n^t ^f plifiinjjfiT com- 



binaiions. If methyl ene blue or alizarin blue are injected 

 into the vein of an animal, the blood becomes blue, but the 

 muscles remain colourless, having deoxidised the pigment 

 and reduced it to a colourless condition. When freely ex- 

 posed to air after death, the blue colour returns. 



3. Changes in Blood passing through Muscle. Observa- 

 tions on excised muscle carry us no further, but by ascertain- 

 ing the changes in composition which the blood undergoes 

 in passing through a muscle, further information may be 

 gained. 



For this purpose the two hind limbs of a dog have been 

 used. The blood going to one leg, and the blood coming 

 from the other, are collected at the same time. It is found 



