3H THE VASCULAR SYSTEM 



partial heart-block, or may produce no alteration at all in the 

 heart rhythm. 



Although the heart rhythm normally commences at the 

 sino-atrial node and is conveyed to the ventricle by the 

 atrio-ventricular bundle, a complete lesion of the latter does 

 not necessarily involve cessation of the ventricular contractions. 

 Under these circumstances, ventricular contractions continue 

 but they acquire a rhythm of their own, which is slower than, 

 and quite distinct from, the atrial rhythm. Pulse tracings of 

 this condition show that a ventricular contraction does not 

 necessarily succeed an atrial contraction, but the two may 

 occasionally be synchronous or the ventricular systole may 

 precede the atrial systole. 



THE GREAT VESSELS 



The Superior Vena Cava is formed by the union of the 

 right and left innominate veins, which unite with one another 

 behind the sternal end of the first right costal cartilage. It 

 descends, partly behind the sternum and partly projecting 

 beyond its right border, to terminate in the uppermost part of 

 the right atrium. Below the second costal cartilage, the vena 

 cava is enclosed within the fibrous pericardium and it is 

 related anteriorly to the serous pericardial sac. When the 

 dulness to percussion produced by a pericardial effusion 

 extends upwards into the right second intercostal space, the 

 superior vena cava is usually compressed and the veins of the 

 head, neck and upper limbs become greatly engorged. A 

 similar condition is met with when the vena cava is com- 

 pressed by an aneurism of the ascending aorta, which lies to 

 its left side and on a slightly anterior plane. Such an 

 aneurism may not only compress the vena cava but may 

 subsequently rupture into it, giving rise to an arterio-venous 

 aneurism, which is indicated by a sudden great increase in the 

 already existing venous engorgement. 



The Innominate Vein is formed behind the sternal end of 



