414 THE RESPIRATION 



come active in the case of a salivary gland whose cells have been para- 

 lyzed by atropine, but it is a significant fact that this dilatation is of rela- 

 tively short duration, whereas that produced by glandular activity lasts 

 for some time. The suggestion seems therefore not out of place that un- 

 der normal conditions the initial dilatation of an acting gland may be 

 brought about through nervous stimuli, but the later dilatation is main- 

 tained by metabolic products, and by rise in temperature. 



It is probable that the increased blood flow acting along with the 

 accelerated dissociation of oxyhemoglobin is adequate to account for 

 all of the increased consumption of oxygen by the active muscles. The 

 oxygen simply diffuses into the muscle fibers from the blood plasma. 

 It has commonly been supposed that the avidity of the muscle for oxygen 

 is so great that the tension within the fiber immediately falls to zero 

 but Krogh has brought forward evidence to show that this is not neces- 

 sarily the case. This worker has shown, by microscopic examination, that 

 the capillaries containing blood are relatively scanty in a resting muscle 

 being, however, uniformly distributed around the fibers, but that many 

 additional capillaries become filled with blood and make their appearance 

 during activity. That is to say many capillaries that are empty during 

 rest open up and fill with blood when the muscle becomes active. He has 

 also shown by mathematical calculations that every part of the muscle 

 fiber must be readily accessible to oxygen molecules conveyed into them 

 purely by physical processes. 



