312 THE VASCULAR SYSTEM 



ceeded by a marked fall in the pressure, since, during 

 ventricular systole, the atrial walls are relaxed and the atrial 

 capacity is further increased by the movement of the atrio- 

 ventricular base towards the apex. The filling up of the 

 atrium causes a rise in the intra-venous blood-pressure which 

 is accentuated by the return of the atrio-ventricular base to 

 the position of rest at the end of ventricular systole. The 

 phase ceases when the tricuspid valve opens (Fig. in, v) 

 and it is succeeded by atrial systole. This description only 

 refers to the character of the venous pulse so long as the 

 tricuspid valve is competent. 



In tricuspid incompetence, the right atrium has to deal 

 with a larger quantity of blood and the pressure during atrial 

 systole therefore rises to a higher level. Ventricular systole 

 is accompanied by a fall in the pressure, but the succeeding 

 secondary rise is hastened in its occurrence by the regurgitation 

 into the atrium. Further, the fall in the secondary wave is 

 less marked, as blood passes from the atrium into the ventricle 

 during the whole of the diastolic period. This variety is 

 referred to as the ventricular type of venous pulse. 



The Heart Rhythm. In the primitive tubular heart, the 

 rhythmical waves of contraction begin at the sinus venosus 

 and pass to the atrium and ventricle by direct continuity of 

 muscle fibres. In the adult heart the sinus venosus is in- 

 corporated in the atria and most of the muscle fibres are 

 interrupted at the atrio-ventricular base. It is believed that 

 atrial contraction begins near the orifice of the superior vena 

 cava in a specialised area, with distinctive histological 

 characters, which has been termed the sino-atrial node. The 

 wave of contraction passes by muscular continuity to the left 

 atrium and it reaches a second specialised area, termed the 

 atrio-ventricular node, which is situated on the inter-atrial 

 septum near the atrio-ventricular base. From this node the 

 atrio-ventricular bundle (of His), which is a collection of 

 specialised muscle fibres, takes its origin, and it extends 

 across the atrio-ventricular base into the interventricular 



