16753 str (87)

104 MUSCLE AND RUBBEeI

relaxing chains. The relative proporti_0n^®

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depends on the type of muscle in quest‘ m tw,

so-called | liąuid ” linkages having been^^ł

into linkages of much higher viscositv un-+?^Verted

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tracted muscle loses this rubber-like s+r, ⁿ‘ lii *< S§&IIRR I i.-„i—li

this the early simple models of muscle as an^ ii spring operating in a truły fluid medium (HiU or later, as a series of springs, some of which onlv^

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taneous curling up of the chains. Ón ar/

damped by a truły fluid materiał (Levin and Wvm ' 1927), were shown to be inadeąuate. Stevens and Metcalf suggest that the power output of the muscle (force X velocity) is constant during changes in strain; but Fenn and Marsh's experiments show that the best empirical eąuation for stress dissipation is of a log/log type, analogous to the Ostwald eąuation already discussed. Meyer and Picken have madę an extended study of the thermo-elastic properties of muscle. They verify the earlier hypothesis that the structure is very similar to that of rubber. For smali and large deformations the elasticity-temperature coefficient is negative, as is usual for solids ; but at intermediate deformations it is positive. These authors give an explanation of the elastic phenomena very similar to that of Mark, which has already been discussed. They conclude that muscle consists of “flexible protein chains forming a three-dimensional network and free chains in the meshes of this net.” X-ray diagrams show, as with rubber, a weak structure for the strained, but not for the unstrained materiał.

A great deal of work has been done on the peculiar properties of rubber, apart from that done by Mark. The work of Dillon has already been mentioned, and that of Whitby is worth studying. Eccles and Thompson have developed a thermodynamic theory

f visco-elasticity, which is claimed to work satis-factorily when it is applied to rubber strands. Griffith has also put forward an interesting theory ₀f rubber structure. His idea is that rubber is a _network of long chains, the junction points in the chains being joined to their neighbours. The chains themselves are in rotation about an axis joining adiacent junction points. The centrifugal force causes a tension in the swinging molecular chains, which behave rather hke a series of skipping ropes. The Joule effect and the elastic properties of rubber are explained on the basis of this theory. Mack has a theory of folded chains, reminiscent of Astbury's ideas on fibres.

Szegvari describes a phenomenon in certain sols that is called " flow-elasticity ” (Fliesselastizitat), and Scott Blair has found the same thing in fibrous honeys. When the materiał is flowing through a capillary tubę under pressure, if the pressure is suddenly released, the materiał not only stops flowing, but actually flows backwards a little. Szegyari defines this phenomenon in terms of a limiting stress (i.e., a yield-value), not as a modulus; and his eąuation is thus similar to that of Bingham.

BIBLIOGRAPHY

Andrade and Tsien. Proc. Roy. Soc. (A), 1937, CLXIII., 1. Astbury. Many papers in the Proceedings of the Royal Socieły, best summarised in his book, “ Fundamentals of Fibrę Structure,” 1933. Oxford University Press.

Becker. Zeits.f. Phys., 1925, XXXIII., 185.

Birch. J. Appl. Phys., 1938, IX., 279.

Bohn and Bailey. Cer. Chem., 1936, XIII., 389, 560. Boltzmann. Pogg. Ann. der Phys., 1876, VII., 624.

Chalmers. Proc. Roy. Soc. (A), 1936, CLVI., 427.

Clayton and Peirce. J. Tex. Inst., 1929, XX., 3*5-Davis. Dairy Science, 1937, VIII., 245.