THE CIRCULATION 231 



We have said that the heart is so formed that no vibrating 

 fluid vein is produced when it is functioning normally. Mur- 

 murs are due to alterations in the valves which are visible 

 after death. This statement needs modification. Not in- 

 frequently functional murmurs are heard, which disappear 

 again after a time in a few weeks, or even days, perhaps. The 

 explanation of murmurs of this class is very difficult. They 

 are heard most frequently in anaemic persons, and appear in 

 these cases to be due to the heart having shrunk, owing to the 

 blood in circulation being deficient in quantity, until the cavities 

 of the ventricles have a smaller diameter than that of the 

 great arteries into which they expel their contents. 



Such is the explanation of the physical cause of murmurs 

 given by Chauveau and Marey, the physiologists who have paid 

 most attention to this subject. But it must be remembered 

 that the valves which, when diseased, are the sources of the 

 murmurs are membranous structures. It may be that fluid 

 veins would be produced by them if they were rigid ledges 

 which jutted into the blood-stream ; but, being membranous, 

 they are capable of vibration. Certain physicists are of opinion 

 that a murmur is caused, not by the vibration of a fluid vein, 

 as such, but by the vibration of the membranous structure 

 which impedes the passage of the fluid. The physics of the 

 problem is of little consequence to the physician. The murmur 

 is produced at the spot where a diseased valve is situated, and 

 is propagated forwards. It enables him to ascertain with 

 accuracy what is amiss with the heart. 



Bloodvessels. The greater circulation occurs through a 

 closed system of vessels which unite the left ventricle with the 

 right auricle. The aorta gives off lateral branches. Its 

 branches branch. Subdivision continues until the vessels are 

 just wide enough to allow blood-corpuscles to pass in single 

 file, or but little wider. When a bough of a tree divides, the 

 united cross-sections of its twigs, their soft bark being stripped 

 off, may be a little larger than the cross-section of the bough ; 

 but the disparity is usually small. The united cross-sections of 

 the smaller arteries is considerably greater than that of the 

 trunks which give origin to them. By the time the capillaries 

 are reached, their total bed their united cross-section is about 

 640 times as great as that of the aorta. This estimate is based 



