THE CIRCULATION OF THE BLOOD. 261 
THE BEAT OF THE HEART AND ITS MODIFICATIONS. 
As has been already noted, the cardiac muscle has features 
peculiar to itself, and occupies histologically an intermediate 
place between the plain and the striped muscle-cells, and that 
the contraction of the heart is also intermediate in character, 
and is best seen in those forms of the organ which are some- 
what tubular and beat slowly. But the contraction, though 
peristaltic, is more rapid than is usually the case in other 
organs with the smooth form of muscle-fiber. 
The heart behaves under a stimulus in a peculiar manner. 
The effect of a single induction shock depends on the phase of 
contraction in which the heart is at the moment of its applica- 
tion. Thus at the commencement of a systole there is no visi- 
ble effect, while beats of unusual character result at other 
times. But tetanus can not be induced by any form or method 
of stimulation. The latent period of cardiac muscle is long. 
In a heart at rest a single stimulus (as the prick of a needle) 
usually calls forth but one contraction. 
THE NeERvous SysTEM IN RELATION TO THE HEART. 
The attempts to determine just why the heart beats at all, 
and especially the share taken by the nervous system, if any 
direct one, are beset with great difficulty; though, as we shall 
attempt to show later, this subject also has been cramped within 
too narrow limits, and hence regarded in a false light. 
Till comparatively recently the frog’s heart alone received 
much attention, if we except those of certain well-known mam- 
mals. In the heart of the frog there are ganglion-cells in vari- 
ous parts, especially numerous in the sinus venosus (or expan- 
sion of the great veins where they meet the auricles); also in 
the auricles, more especially in the septum (ganglia of Remak), 
while they are absent from the greater part of the ventricle, 
though found in the auriculo-ventricular groove (ganglia of 
Bidder). 
Recently it has been found that ganglion-cells occur in the 
ventricles of warm-blood animals. In the hearts of the dog, 
calf, sheep, and pig, which are those lately subjected to investi- 
gation, it is found that the nerve-cells do not occur'near the 
apex of the ventricles, but mainly in the middle and basal por- 
tions, being most abundant in the anterior and posterior inter- 
ventricular furrows and in the left ventricle. But there are 
