74 A MANUAL OF PHYSIOLOGY 



at every stroke of the pump in exactly the quantity in which it enters, 

 for water is practically incompressible, and the total quantity present 

 at one time in the system cannot be sensibly altered. In the 

 intervals between the strokes the flow ceases ; in other words, it is 

 intermittent. It is very different with a system of distensible and 

 elastic tubes. During each stroke the tubes expand, and make 

 room for a portion of the extra liquid thrown into them, so that a 

 smaller quantity flows out than passes in. In the intervals between 

 the strokes the distended tubes, in virtue of their elasticity, tend to 

 regain their original calibre. Pressure is thus exerted upon the 

 liquid, and it continues to be forced out, so that when the strokes of 

 the pump succeed each other with sufficient rapidity, the outflow 

 becomes continuous. This is the state of affairs in the vascular 

 system. The intermittent action of the heart is toned down in the 

 elastic vessels to a continuous steady flow. 



The Beat of the Heart. In the frog's heart the contraction 

 can be seen to begin about the mouths of the great veins 

 which open into the sinus venosus. Thence it spreads in 

 succession over the sinus and auricles, hesitates for a 

 moment at the auriculo-ventricular junction, and then with 

 a certain suddenness invades the ventricle. In all prob- 

 ability the contraction wave is propagated without the 

 intervention of nerves, from fibre to fibre of the muscular 

 tissue, which, although presenting certain variations in its 

 character in the different divisions of the heart and at their 

 junctions, forms a more or less continuous sheet over the 

 whole of the organ. This conclusion rests in part upon the 

 observation that the delay of the wave at the auriculo- 

 ventricular groove is much greater than it ought to be if the 

 excitation were transmitted by nerves, since the velocity of the 

 nerve-impulse is exceedingly great (p. 582) ; and the further 

 observation that, when the ventricle is caused to contract 

 by artificial stimulation of the auricle, this delay is appre- 

 ciably greater when the stimulus is applied as far from the 

 ventricle as possible than when it is applied as near to it as 

 possible. In the mammalian heart the starting-point of the 

 contraction is likewise the mouths of the veins opening into 

 the auricles, which are richly provided with muscular fibres 

 akin to those of the heart. But the wave advances so 

 rapidly that it is difficult, if not impossible, to trace in its 

 course a regular progress from base to apex, although the 

 ventricular beat undoubtedly follows that of the auricle, and 



