ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 101 



first the muscle is always so firmly contracted, that it not only 

 resists the strong traction of the uninjured elastic ligament, but 

 will even support a weight of more than 20 grs. without visible 

 extension. Even when the shells gape sufficiently, after a long 

 interval, to make effective excitation practicable, the efforts of the 

 weighted muscle to shorten are still considerable, as shown by the 

 fact that each decrease of its load is followed by a corresponding 

 shortening. Even after many hours the presence of a certain 

 " tonus " may generally be demonstrated. As soon as the 

 insertion of the still living muscle is freed on one side, it con- 

 tracts quickly to less than half its length with completely closed 

 shell. In time, of course, this tonus diminishes slowly. If 

 a preparation is left for several hours at medium temperature, 

 the gradual relaxation can be easily determined. While at 

 the beginning it takes considerable force to separate the two 

 halves of the shell, this becomes gradually easier, and after 

 several hours a weight of hardly 10 grs. will sometimes pro- 

 duce almost maximal extension of the muscle. When, therefore, 

 the elastic ligament has not been injured in preparation, the 

 shells, which were tightly closed at first, gape wider and wider, 

 because the ratio between the tension in the opening ligament 

 and the tonic effort of the muscle to contract, alters constantly 

 in favour of the former. 



The decrease of tonus, however, begins almost instantaneously 

 if the preparation is submitted to a higher temperature (immersion 

 in H 2 at about 30 C.), which soon effects a considerable relaxa- 

 tion. On subsequent cooling the tonus is only partially restored, 

 though in other smooth muscles it comes back completely (63). 

 Bernstein recently investigated the effect of different temperatures 

 upon the muscles of the frog's stomach, arriving like Griinhagen 

 and Samkowy (64) at precisely the same results obtained by 

 Biedermann from smooth molluscan muscle. Bernstein, after 

 removing the mucosa, took a circular piece of the muscular layer, 

 and stretched it between two hoops in a glass vessel, the shorten- 

 ing, or extension, being conveyed to a writing-lever by means of 

 a thread running over a pulley. The medium of heating was 

 either physiological salt solution, previously brought to the 

 required temperature, and then poured into the vessel, or air 

 saturated with steam. When treated in this way the ring of 

 muscle corresponds exactly with the adductor muscle of molluscs 



