THE CIRCULATION OF THE BLOOD. 265 
in action neighboring heart-cells to contraction. The ability 
that one protoplasmic cell-mass has to initiate in others, under 
certain circumstances, like conditions with its own, is worthy 
of more serious consideration in health and disease than it has 
yet received. 
5. The influence of the cardiac nerves becomes more pro- 
nounced as we ascend the animal scale. Their share in the 
heart’s beat will be considered later. 
6. Apparently in all hearts there is a functional connection 
leading to a regular sequence of beat in the different parts, in 
which the sinus or its representatives (the terminations of great 
veins in the heart) always takes the initiative. One part hav- 
ing contracted, the others must necessarily follow; hence the 
rapid onset of the ventricular after the auricular contraction in 
the mammal, and the long wave of contraction that seems to 
pass evenly over the whole organ in cold-blooded animals. 
The basis of all these factors is to be sought finally in the 
natural contractility of protoplasm. A heart in its most devel- 
oped form still retains, so to speak, the inherited but modified 
Ameeba in its every cell. 
Whether the intrinsic nerve-cells of the heart take any 
share directly in the cardiac beat must be considered as yet 
undetermined. Possibly they do modify motor impulses from 
nerves, while again it may be that they have an influence over 
nutritive processes only. The subject requires further study, 
both anatomical and physiological. 
INFLUENCE OF THE VAGUS NERVE UPON THE HEART. 
The principal facts in this connection may be stated as fol- 
lows, and apply to all the animals thus far examined : 
1. In all cases the action of the heart is modified by stimu- 
lation of the medulla oblongata or the vagus nerve. 
2. The modification may consist in prompt arrest of the 
heart, in slowing, in enfeeblement of the beat, or a combination 
of the two latter effects. 
3. After the application of the stimulation there is a latent 
period before the effect is manifest, and the latter may outlast 
the stimulation by a considerable period. 
4. In most animals the sinus venosus and auricles are af- 
fected before the ventricles, and the vagus may influence these 
parts when it is powerless over the ventricle. 
5. After vagus inhibition, the action of the heart is (almost 
