THE HEART 



247 



said that they pass spirally around all the chambers of 

 the heart. By their united contraction the cavities are 

 diminished and may be well nigh obliterated, although 

 we cannot assume that this happens at each heart-beat. 

 We are not to think that the solid tissues of the heart lose 

 volume when the act of contraction is performed but the 

 organ externally becomes smaller just in proportion as 

 the blood is pressed out of 

 its ventricles. 



The Heart Valves. As 

 the chambers of the heart 

 are rhythmically widened 

 and narrowed, the blood 

 is driven in a fixed direc- 

 tion. This could not be 

 the case if it were not for 

 the two sets of valves with 

 which the organ is pro- 

 vided. If we speak of 

 those on the left side the 

 description will answer ?IG 54 ._ For the sake of clear _ 



approximately for those ness the two halves of the heart are 



On the right. We find a separated by an interval (r.a.) and 



, , , , . , (l-a.) are the right and left auricles; 



valve between the auricle ( r . l) .) an d (i. v .) are the right and left 



and the ventricle and an- ventricles; (1) is the tricuspid, (3) 



other valve or set of valves tales' 1 ' (2) "^ (4) ^ semihmar 

 in the root of the great 



artery. The valves are membranous flaps with no 

 power to move save as the currents of blood actuate them. 

 On the left side the valve guarding the opening from the 

 auricle to the ventricle is called the mitral valve. Its 

 fellow on the right is the tricuspid. 



Students often find it hard to remember which is on 

 the right and which on the left. Huxley's mnemonic 

 device may be quoted (without endorsing the statement) : 

 "a mitre is a bishop's crown and a bishop would never be 

 found on the right side." The mitral valve consists of 

 two thin and transparent but very tough sheets of tissue 



