68 General Notes. 
mammals the contractions of the heart muscle are always maximal, 
and a rhythmic rise and fall occur in the muscular excitability— a 
fall immediately succeeding the contraction, followed by a gradual 
rise during the phase of relaxation ; hence the more rapid the beat, 
the less powerful it is, and vice versa. Constant currents and weak 
induction currents are alike in causing an acceleration of beat in an 
already active heart, and the appearance of a rhythmic series of beats 
in one previously quiescent. As in all other Vertebrates, the mam- 
malian heart-beat partakes of the nature of a progressive contrac- 
tion beginning at the venous end of the organ. The place of origin 
of the contraction seems to be the walls of the great veins, and 
the time of origin is the same for the venje cavge and the pulmonary 
veins. Whether the mammalian cardiac ihythm is nervous or 
myogenic — i.e., whether it is a property of nervous or muscular 
tissue — is impossible at present to decide. That heart muscle has 
a spontaneous rhythmic power of its own is abundantly proved for 
Fishes, Amphibians, and Eeptiles, although it seems that normally, 
in the auricular muscle at least, such power is in abeyance. The 
following facts concerning the mammalian heart muscle will help 
in the future to elucidate this subject. All paits of the heart 
are endowed with independent rhythmic power, as is proved by the 
continuance of rhythmic contractions in parts separated from each 
other — e.g., in ventricles separated from auricles ; the independent 
ventricular rhythm seems at times to be myogenic, for by increas- 
ing the local excitability, as by the local application of heat, the 
contraction may be made to begin in the apex, where, according to 
the researches of the author and those of Kasem Beck, nerve cells 
do not occur ; the automatic rhythmic property is not equal in the 
various parts of the heart, being apparently highest in the venous 
terminations, and lowest in the ventricle, as indicated, among other 
things, by the slow rhythm in the isolated ventricle (which is 
in harmony with what exists in the lower Vertebrates) ; the rhythm 
originating at the venous terminations apparently dominates the 
whole heart, and determines the rate of its action ; hence the causes 
determining the rhythm of the intact heart are to be sought for at 
the venous end of the organ. The usual order of contraction may 
be altered and even entirely reversed by artificially stimulating a 
portion of the surface — e.g., stimulation of the ventricle is followed 
by contraction of the ventricle, then auricle, then venous termina- 
The question of the mode of propagation of the normal contrac- 
tion over the auricles and ventricles is discussed by the author at 
some length, in view of the fact that Gaskell has urged tiiat in the 
tortoise the phenomenon is simply one of muscular conduction. 
as regards the passage from auricle to ventricle at least, by the fact 
that here is a distinct break in the muscular continuity, the auricles 
and ventricles being separated by a considerable amount of connec- 
tive tissue. It seems impossible to account for the sequence on 
