MUSCULAR ACTION 383 



It is in part the goal of biochemistry and of chemical physiology to 

 replace this possible outline with the actual formula in all its details. 



Carbohydrates are present in muscle only in a small percentage, as 

 we have seen, but their importance in the metabolism of this tissue is 

 apparently great. The two chief carbohydrates present are glycogen 

 and, in much smaller proportion, dextrose. The former, glycogen, 

 present in most animal tissues, if not all, is doubtless the chief source 

 of muscular energy. Manche found that a limb made to contract for 

 about an hour showed about 14 per cent, less glycogen than the corre- 

 sponding resting limb, and that ligation of the arteries produced a like 

 result. In starvation the glycogen rapidly disappears from the muscle. 

 In muscular paralysis, whatever its cause, glycogen accumulates in this 

 tissue. Most abundant of the elements in the inorganic salts of muscle 

 are potassium (which is preponderant), sodium, calcium, magnesium, 

 chlorine, sulphur, and phosphorus, the three last being represented as 

 the chlorides, sulphates, and phosphates of the others. It is a striking 

 fact that a muscle does not contract spontaneously in aqueous solutions 

 containing no ions, for example, in a distilled-water solution of chemically 

 pure cane-sugar. The large amount of research which has recently 

 been made into the relations of muscular activity and inorganic salts is 

 left to tell us what chemism and electricity have to do with muscular 

 action. Urea, creatin, creatinin, xanthin, hypoxanthin, and carnin are 

 found in muscle, as well as many other nuclein bases. Just how these 

 substances are produced in the katabolism of muscle it is impossible at 

 present to say. 



The Modes of Action of Muscle. It is one thing to understand the struc- 

 ture and composition of a complicated mechanism, but quite another 

 sometimes to know exactly how it works. In the case of muscle this 

 problem is made worse by the minuteness of the structures and by the 

 intricacy of the physics concerned in the action of this highly differ- 

 entiated kind of protoplasm. As yet we cannot tell how muscle develops 

 its contractile energy, so the best we can do is to describe summarily a 

 few of the various theories as to the matter. Our immediate problem 

 is, then, as to exactly how and why the fibrils of muscle shorten and 

 thicken and then immediately lengthen and attenuate again. Do cross- 

 striated and smooth muscle act in the same way? (See also page 487.) 



We can go a certain way on relatively solid ground. We know the 

 probable structure of cross-striated muscle, substantially, so far, at least, 

 as appearances go. We know that it consists of two sorts of substances, 

 one (anisotropic) doubly refracting polarized light, the other (isotropic) 

 refracting it singly. We know that when the contraction occurs in 

 cross-striated muscle the latter kind of material changes its place some- 

 what, while the former kind does not do so. We are sure that the 

 metabolism of all sorts of muscle is, in part, the oxidation of carbo- 

 hydrates and of protein, sarcolactic acid being a way-product, and 

 carbon dioxide and water among the end-products. The more active 

 the contraction of the muscle the more oxygen it consumes and the 



