The Mechanics of the Blood Vascular Sustem of Ascidiella Aspersa

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Abstract
1. The reversal of the tunicate heart involves two problems: first, why it stops beating in a particular direction; and secondly, why having stopped it should then reverse. 2. Two types of theory have been put forward, one based on the idea that a ‘back pressure’ is gradually built up which compels the heart to stop, and the other depending on opposing centres of contraction at each end of the heart taking control alternately. 3. The present paper provides quantitative evidence, based on Ascidiella aspersa (O. F. Mueller), that a back pressure is built up. The faster the heart beats the more frequently reversals occur, a back pressure building up more rapidly when the heart beats quickly. 4. Various criticisms of the back-pressure theory are considered and discussed. 5. Examples are given of other closed contractile systems containing fluid which show reversal, the tunicate heart being by no means unique.

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