Page:Popular Science Monthly Volume 75.djvu/67

Rh instance that of Salpa, is a muscular tube over which peristaltic waves run from end to end. As is well known, the direction of these waves reverses from moment to moment, running for a short interval toward the visceral end of the heart, advisceral waves, and then forward the respiratory end, abvisceral waves. In Salpa africana-maxima, to take a single instance, according to Schultze (1901), after 16 abvisceral waves had passed over the heart in some 20 seconds, a resting period of

2 seconds ensued, whereupon 18 advisceral waves occupying 25 seconds preceded another resting period, etc. When the heart is removed from the body of a Salpa, it continues to beat with characteristic reversal. Stimulation of the central nervous ganglion of a normal Salpa has no effect upon the heart-beat, and though a removal of this organ is followed by a reduction in the rate, the same reduction is to be observed when other parts of the body than the central nervous organ are cut

out. Small fragments of the heart of Salpa also beat rhythmically when entirely isolated, a fact recently confirmed by Hunter (1903) on Molgula, and a most careful search of these fragments has failed to reveal nerve-cells or nerve-fibers. It seems therefore clear that the rhythmic heart-beat of the tunicates is myogenic in origin. This seems also to be true of the embryonic, vertebrate heart, for His (1891) has shown that this organ beats at a time when no trace of nervous tissue can be discovered in it.