Von Uexküll1 has discovered that Mollusca have two types of muscle, a motor muscle which closes the shell and another which holds, with enormous power and without output of energy, the previously closed shell. He could demonstrate different organs for these two functions also in other organisms of a lower order. He differentiates these two functions as motor and locking (Sperr—) functions. Von Uexküll believes that in other animals, in which only one motor organ can be demonstrated, both functions are united in this organ. In Sipunculus, the muscle not only can shorten and lengthen but also keeps a certain level of tone at the same time, which can be varied independently. Certainly these are two functions of a single organ but it is doubtful whether they can be identified with the motor and the "locking" function of the shell muscle. This has been taken for granted, however, by
SCHALTENBRAND G. MUSCLE TONE IN MAN. Arch Surg. 1929;18(4):1874–1885. doi:10.1001/archsurg.1929.01140130976065
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