RESPIRATION BEYOND THE LUNGS 409 



In the order of their oxygen requirements, or the coefficient of oxida- 

 tion, as it is called, the tissues may be divided into four groups ; glandular, 

 muscular, connective, and nervous. The nervous tissues should possibly 

 stand above the connective, but very little is known regarding their 

 oxygen consumption, although it appears that this is quite low (Hill and 

 Nabarro). It is of course necessary in making these comparisons to 

 secure the coefficient of oxidation both when the tissue is at rest and 

 when it is thrown into varying degrees of activity. Special attention 

 has been devoted to the requirements of skeletal muscle, heart muscle 

 and the salivary glands. 



Skeletal Muscle. It will be seen from the table that a resting muscle 

 while still connected with the nervous system consumes about 0.006 c.c. 

 2 per gram per minute; a small amount when compared with other 

 tissues. The consumption increases by from ten to fifteen times during 

 muscular exercise. This increase depends partly on increased blood 

 flow. 



As a type of the experimental method by which these values are ob- 

 tained and to show for what purpose the muscle requires the extra oxy- 

 gen when it contracts it will be of interest to consider the observations 

 of Verzar 47 . This worker isolated the gastrocnemius muscle of the cat, 

 and without disturbing its blood supply collected samples of blood by 

 introducing a 1 c.c. pipette into a branch of the saphenous vein. Activ- 

 ity was induced by throwing the muscle into brief tetanus by the ap- 

 plication of an electrical stimulus to the sciatic nerve. During its con- 

 traction the muscle lifted a weight, so that it did about 70 gram-centi- 

 meters of work at the beginning of each period of tetanus. The velocity 

 of bloodflow was determined by the rate at which the blood flowed along 

 the pipette, and the 2 -consumption, by the difference in percentage of 

 2 in the venous and the arterial blood. These measurements were made: 

 (1) before contraction, (2) during contraction, and (3) after contraction. 

 It was found that although the 2 consumption was usually somewhat 

 greater during the tetanus than during rest, the most outstanding and 

 significant result was that a, great increase occurred immediately fol- 

 lowing the tetanus that is, the call for 2 continues for some time after 

 the actual work has been performed. This result shows that the con- 

 traction is not dependent upon oxidation, but that the oxidation occurs 

 mainly after the contraction is over. The mechanism involved in mus- 

 cular contraction can not therefore be analogous with that by which 

 energy is liberated in a steam engine by the oxidation of the fuel. 



Interesting results corroborative of these conclusions have been se- 

 cured by observations on the heat production of isolated muscles. It 

 was found that heat production occurred after a single shock to the 



