CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 773 



muscular work have been previously given (p. 612), and need not 

 be repeated here. It may be added that the various food substances 

 yield muscular energy in isodynamic relation. In other words, a 

 given amount of muscular work requires the expenditure of approxi- 

 mately the same quantity of chemical energy, whether it comes 

 almost entirely from protein, or chiefly from carbo-hydrates, or 

 chiefly from fat. Some observers have stated that the taking of 

 even a comparatively small quantity of sugar vastly increases the 

 capacity for muscular work as measured by the ergograph (p. 750). 

 But although it is not to be doubted that sugar is under normal 

 circumstances one of the most important substances used up in 

 muscular contraction, the claim that sugar is, par excellence, the 

 food for muscular exertion has not yet been made out. 



Physico-Chemical Conditions of Muscular Contraction. For excised 

 fresh muscle A (p. 427) has been estimated at 0-68 C. But this is 

 probably higher than in the living body, for after excision waste products, 

 with their relatively small and numerous molecules, are still for a time 

 produced, and are no longer removed by the blood. In salt solutions 

 isotonic with the muscle substance e.g., for the frog's gastrocnemius 

 at room-temperature a 0-75 per cent, solution of sodium chloride the 

 resting muscle neither gains nor loses water for some hours. The active 

 muscle behaves quite differently. When a muscle immersed in isotonic 

 salt solution is tetanized, water enters it, leading to an increase in 

 weight and a diminution in specific gravity (Ranke, Loeb, Barlow). 

 The same occurs even when blood is circulated through active muscles, 

 the blood becoming poorer in water (Ranke). This may be explained 

 by the increase of osmotic pressure in the muscle substance which must 

 accompany the decomposition of large molecules into small. As fatigue 

 progresses, a movement of water in the reverse direction occurs, and 

 the muscle rapidly loses water. Exposure of the fatigued muscle for a 

 sufficient time to an atmosphere of oxygen restores the osmotic proper- 

 ties of the resting muscle. Striking differences have also been demon- 

 strated in the behaviour of resting and fatigued muscle to hypotonic 

 solutions or water. Hales observed long ago that, on injecting large 

 quantities of water into the bloodvessels of a dog, so as to replace the 

 blood, marked swelling of the muscles occurred. This physiological fact 

 is well known to the pork-butchers in China, who have given it a 

 practical, if not a very praiseworthy, application in sophisticating their 

 product by increasing its weight (MacGowan). 



So long as the muscular fibres are uninjured they are permeable or 

 impermeable for exactly the same compounds as other animal and 

 vegetable cells. All substances easily soluble in media like ether or 

 olive oil readily penetrate them (Overton). To most salts they are 

 relatively impermeable, as is shown by the fibres retaining their original 

 volume in isotonic solutions of them. In particular, they cannot easily 

 take up or retain the salts of the blood-plasma, otherwise the observed 

 qualitative differences e.g., the preponderance of potassium in the 

 muscle and sodium in the plasma could not be maintained. There are 

 facts which indicate that temporary changes in the permeability to ions, 

 not only of muscular fibres, but also of nerve fibres and other excitable 

 structures, are concerned in their stimulation. Potassium salts after a 

 time seem to produce an effect upon frog's muscle, which alters its 

 permeability so that it takes up water from hypertonic solutions. 



