RESPIRATION BEYOND THE LUNGS 413 



of 2 ; and (2) by an increase in the mass movement of blood through 

 the vessels of the acting tissue. 



Regarding the first of these possibilities, there is no doubt that acids 

 are produced during metabolism of acting tissues. As we have seen, 

 when muscles contract in the presence of an abundance of 2 , C0 2 is 

 produced in large amounts, and when they contract in a deficiency of 2 , 

 sarcolactic acid. In the submaxillary gland, too, it has been possible to 

 show that the C H of the venous blood, as measured by the value of K of 

 the dissociation curve of hemoglobin, becomes distinctly increased dur- 

 ing glandular activity. That this increase in C H will dislodge 2 we have 

 already seen (page 401). 



That it should have been impossible by direct methods to show any 

 change in C H of the blood as a whole during muscular exercise (cf. page 

 410) does not necessarily indicate that such may not occur in the capillary 

 blood of the muscles themselves. There is considerable indirect evi- 

 dence that CH rises in the blood circulating through the muscles. This 

 increased acidity will greatly facilitate the unloading of 2 from the 

 blood; not so much because it depresses the level of the dissociation 

 curve as that it accelerates the rate of dissociation (page 399). This 

 acceleration will be further encouraged by the rise in temperature of 

 the blood (pages 409 and 433). 



In connection with dilatation of the blood vessels of the active tis- 

 sue it is most important to bear in mind that this may occur either 

 in the arterioles or in the capillaries or, of course, in both together. 

 Krogh 42 has conclusively demonstrated that the capillaries of muscles 

 can become dilated during activity quite independently of dilatation 

 of their contributory arterioles and it has been shown by Dale and 

 Richards 43 that histamine causes capillary dilatation accompanied by 

 arteriole constriction. The application of this discovery in the patho- 

 genesis of shock has already been referred to (page 307) and it is pos- 

 sible that histamine, or a similar substance, may be the cause of the 

 capillary dilatation during muscular activity. 



But before such an hypothesis can be entertained, it is necessary to 

 show that, independently of nerve impulses, the blood vessels of an acting 

 organ may dilate. The best evidence has been secured by studying the 

 effects of stimulating with epinephrine the cervical sympathetic nerve to 

 the submaxillary gland of a cat. The gland cells become more active, 

 and dilatation of the artery occurs, although on blood vessels alone 

 epinephrine in similar dosage produces constriction. Of course in show- 

 ing that local chemical products of activity serve as the excitant of local 

 dilatation, we do not mean to imply that the vasodilator fibers going to 

 the blood vessels are of no use. Indeed we know that such fibers do be- 



