PHYSIOLOGY OF SMOOTH AND STRIATED MUSCLE 535 



stituted for the NaHCOs, are particularly interesting, because 

 it is not improbable that lactic acid may play an important part 

 in the physiology of the tissue. It has been shown on p. 525 that 

 small quantities of lactic acid inhibit the tendency of smooth 

 muscle to gain weight in Ringer's solution. The tendency for 

 the fibers to lengthen is also inhibited by the acid (Experiments 

 27 and 28). 



There are certain cases in which the rule that smooth muscle 

 lengthens when it increases in weight and shortens when it de- 

 creases in weight does not hold. One of these has already been 

 described in a previous article. ^^ Pieces of smooth muscle im- 

 mersed in slightly alkaline Ringer shorten without undergoing 

 SbD.y marked decrease in weight. It has been shown, however, 

 that under these circumstances the fibers lose fluid which is held 

 by the muscle in the interstitial spaces. This case would not, 

 therefore, be an exception to the rule that the shortening of smooth 

 muscle fibers is accompanied by a decrease in their own volume. 



Another partial exception is the case of muscle immersed in 

 Ringer without NaHCOs to which moderately large amounts of 

 lactic acid have been added — 0.05 per cent or above. Pieces of 

 frog's stomach muscle take up considerable quantities of fluid 

 from such acid solutions without lengthening in proportion, 

 though some lengthening often does occur (Experiment 24). 



The action of such lactic acid solutions on smooth muscle is, 

 however, rather complicated. Solutions which contain less than 

 0.025 per cent of acid create a tendency for the muscle fibers to 

 lose weight and shorten. It is probable that when a piece of 

 muscle is immersed in one of the stronger acid solutions, the acid 

 reaches the inner fibers at first in a lower concentration and causes 

 in them a tendency to shorten. Further, the acid produces a 

 peculiar change in the consistency of the muscle, rendering it 

 stiff and inextensible. It may well be that this setting of the 

 muscle substance renders it incapable of undergoing any marked 

 change in length. 



' Meigs; American Jour. Physiol., 1912, vol. 29, p. 317. 



