50 



ELEMENTARY THYSIOLOGY 



tents of the ventricles pass easily into the aorta and 

 pulmonary artery, while none can be made to tiavel the 

 other way from the arterial trunks to the ventricles. 



6. The Beat of the Heart. — Like all other muscular 

 tissues, the substance of the heart is contractile ; but, un- 

 like most muscles, the heart contains within itself a some- 



Fio. 10. — The Orifices OF the Hkart sken from above the Auricles 

 AND Great Ve.ssels being cut awav 



P. A. pulmonary artery, with its semilvin.ar valves ; Ao. anrta do. 



R.A. r. right auriculo-ventricular orifice with the three flaps (t. v. 1, 2, 3) 

 .of tricuspid valve. 



L.A. V. left auriculo-ventricular orifice, with m.v. 1 .and 2, flaps of 

 mitral valve ; h, style puiscd into coronary vein. On the left part of 

 L.A. v., the section (if the auricle is carried through the auricular aji- 

 pondage ; hence the toothed appearaiiee due to the portions in relief 

 cut across. ' 



thing which causes its different parts to contract in a 

 definite succession and at regular intervals. The whole 

 of the heart is not alike in its faculty for contracting 

 spontaneously. Indeed this faculty of initiating the con- 

 traction is normally confined to a quite small jrortion. 

 Starting here the contraction spreads over the auricle by 

 a species of conduction, from the auricle it spreads on 



