ORIGIN OF THE NERVOUS SYSTEM 



63 



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 toward 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 



Fig. 4. Section of a Salpa (modified from Herdman), showing the positions of 

 the atrial aperture (a), branchial aperture (b), digestive tube (d), ganglion (g) 

 and heart (h). 



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 fol- 

 lowed 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 



adv. 



Fig. 5. Heart of a Salpa (modified from Schultze), showing advisceral waves. 

 aov, abvisceral end ; adv, advisceral end. 



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. 



