MUSCLE 671 



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. 399) has been estimated at o'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 diiferently. 

 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 pro- 

 gresses, 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 

 properties of the resting muscle. Striking differences have also 

 been demonstrated 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 preponder- 

 ance 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. Calcium salts have 

 the opposite effect (Loeb) . Sodium (and in a minor degree lithium) 

 salts have a peculiar relation to the contraction of skeletal muscle, for 

 which they appear to be indispensable. Yet sodium chloride produces 

 a paralyzing action on the frog's motor nerve-endings, so that after 

 perfusion with a solution of that salt stimulation of the motor nerve 

 causes no contraction, or with a slighter degree of paralysis con- 

 traction only after a long interval. The effect can be counteracted 

 by solutions containing calcium salts (Locke, Gushing) . 



Rigor Mortis. When a muscle is dying, its excitability, 

 after perhaps a temporary rise at the beginning, diminishes 



