182 MANUAL OF EQUINE MEDICINE. 



Cardiac murmurs are divided into two groups, endocardial 

 and exocardial. 



(A.) Endocardial Murmurs. — Murmurs in the heart are 

 called organic when they depend upon some structural 

 change in the heart, and inorganic when no such change 

 exists. As the heart is continually contracting and dilating, 

 they fall further into two groups, systolic and diastolic 

 murmurs. 



Murmurs in the heart or vessels are due to eddies set up 

 in the blood ; they are heard when a current of fluid passes 

 through a narrow orifice into a larger space. In the heart, 

 so long as the normal relation exists between the size of 

 the orifices and that of the cavities of the ventricles, mur- 

 murs are not produced ; but when the relation is disturbed 

 murmurs are likely to arise. 



They may depend upon dilation of the cavities into which 

 the blood is passing, the orifices being unaltered, or to 

 changes in the condition of the orifices, the cavities remain- 

 ing natural. These conditions are usually associated. A 

 murmur heard during the systole of the heart, called a 

 ventricular systolic murmur, may be due to abnormalities in 

 the auriculo-ventricular valves, or in the arterial orifices. 

 When occurring at the auriculo-ventricular orifice, the mur- 

 mur is owing to the imperfect closure of the valves during 

 the contraction of the heart, so that some of the blood, in- 

 stead of passing through the arterial orifices, passes back, 

 or regurgitates into the auricles. In these cases it is a 

 murmur of regurgitation. 



When occurring at the orifice of the aorta, or pulmonary 

 artery, the systolic murmur is due to thickening or other 

 changes in the semilunar valves causing constriction of 

 the orifice, and thus impeding the onflow of blood. 



In these cases it is a murmur of constriction. 



In the horse the systolic murmur is generally very loud 



