IV] THE BLOOD AND THE ORGANS OF CIRCULATION 63 



Placed ill the course of the lymphatic vessels are the so-calletl 

 lymphatic glands which consist of areolar tissue packed with 

 lymphocytes. When in a position where the vessels connected 

 with them are receiving lymph from an inflamed area these 

 glands tend to become swollen and ma}' cause a considerable 

 amount of discomfort or pain. If owing to one cause or another 

 (local obstruction of vessels or partial failure of the heart to 

 discharge its functions throiigh valvular disease) the lymph 

 increases in the tissues to a quantity beyond the normal, a con- 

 dition of oedema or dropsy is produced. 



The Pulse. The alternate contraction and dilatation of the 

 heart makes itself felt in the arteries as the pulse. "When an 

 artery is situated near the surface of the skin and there is a bony 

 background present, the pulse or pressure wave produced by the 

 heart beat can be recorded by an instrument known as a sphyg- 

 mograph which is placed over the arterj' and responds to the 

 varymg pressure. A tracing can then be obtained by allowing 

 a piece of smoked paper to pass across the recording needle of 

 the instrument, the movements of the needle being regulated 



B 



Fig. .So. Sphvgniograph tracing from facial artery of horse (after Hamilton 

 from Smith, Messrs Bailliere, Tindall and Cox). A before and B after 

 destruction of aortic valves ; in B the dicrotic notch has disappeared. 



by the beats of the pulse. A typical pulse tracing obtained in 

 this way is shown in the ligure. The straight upstroke represents 

 the dilatation of the artery due to the increased blood pressure 

 caused by the contraction of the heart. This is followed by a 

 curved downstroke representing the period of arterial contraction, 

 and this in turn is succeeded by another period of arterial 

 expansion. The downstroke in the tracing is not a continuous 

 curve, but is broken by a slight rise known as the dicrotic wave 

 or notch. This has been shown to be due to the recoil of blood 

 produced by the closure of the semilunar valves in the aorta, for 

 in horses in which these valves have been destroyed cxperi- 



