extrasystolex or interpolated contractions may arise in the vicinity of a
terminal pawmaker (Quincke and Stein, 1932 ; Mislin, 1964 ; Kriebel,
1967 ; Anderson, 1968) and it is possible to create these artificially by
stimulating an end pacemaker in between ordinary contractions. The
existence of such extra systoles suggests that there may be more than
I
I
1
I
I
2
1 0
14
6
m m from end of heart
k”(1. IU. Tho spcwl of ctmtluctrorr iir I rnrn Iimg sc-Krnctntn of tho hoart of C t o r ~ u inlentindim.
Open crrclnx lopresent volocitros in viscoral nrrn of hoart, ~ o l ~ t l
cirolrn volocitios In
b~anchial arm. Tho tlashncl line at 14 mrn rttprexonts tho position nf tho h n d of the
heart. (ltntlmwn from Krinhol, 1970.)
one rentrr of avtivity at the cnd of a heart and that it is possible for
these to beat out of phase and possibly at different frequencies. Kriebel
has in fa& demonstrated that at least two centres occur at each end,
one on either side of the raphe. Arhythmic contractions of the sort
dt?scribecl c m be obliterated by “ driving ” the heart with appropriate
clcctric?ul st,iniuli or by increasing the cardiac pressure. The latter is
equivalent to stretching the myoendothelium ; Anderson reports in-