Position of Cardiac Valves 169 



inter-ventricular groove, giving a transverse branch round the left auri- 

 culo-ventricular groove. 



The coronary arteries give twigs to the large vessels as well as to 

 the auricles and ventricles. When they are diseased (atheroma) they 

 carry an insufficient amount of blood to ttie cardiac tissue, so that 

 fatty degeneration, together with faintness and pain (angina pectoris), 

 result. Should an embolus be carried into one of them, death may 

 immediately occur from paralysis of cardiac muscle. 



The relative position of the chief cardiac orifices. The 

 pulmonary artery and the aorta are developed together : they, there- 

 fore, lie close together ; but the aortic opening is behind the pulmonary 

 because the left ventricle is behind the right. Being close together, 

 the pulmonary orifice must be on the left side of the right ventricle, 

 and the aortic orifice must be on the right side of the left ventricle. 

 The right auriculo-ventricular opening, then, must be to the right of 

 the pulmonary aperture, and the left auriculo-ventricular opening must 

 be to the left of the aortic aperture. (See fig. on p. 168.) 



The situation of the valves. The aortic valves are behind the 

 sternal end of the fourth left space. The pulmonary are a little 

 higher at the junction of the third left cartilage with the sternum. 

 The auriculo-ventricular orifices are behind the sternum at about the 

 level of the third intercostal spaces. 



' Thus these valves are so situated that the mouth of an ordinary- 

 sized stethoscope will cover a portion of them all if placed over the 

 junction of the third intercostal space on the left side with the sternum. 

 All are covered by a thin layer of lung ; therefore we hear their action 

 better when the breathing is for a moment suspended.' (Holden.) 



Occasionally a valve suddenly gives way during violent physical 

 exercise, or as the result of a blow over the front of the chest ; the 

 lesion causes enfeeblement of the circulation and shortness of breath. 



THE SOUNDS OF THE HEART 



The healthy heart-sounds are a long and a short one : lub dup. 

 Then comes a pause which is as long as the short, second sound ; and 

 then lub dup again. We may represent the rhythm by a series of 

 dactyls thus : lub dup pause | lub dup pause. 



And, if we divide the dactyl into eight equal parts, four parts will 

 be taken up by the first sound, two by the second, and two by the 

 pause. Thus : 



. .. 4, 5,6, 7,8 

 1st sound, 2nd, pause 



ist sound, 2nd, pause- 



The two ventricles dilate together and contract together, and the 

 auricles dilate together and contract together. Having divided the 



