258 MANUAL OF PHYSIOLOGY. 



have a transverse, the inner a longitudinal direction. Bands of 

 fibres encircle the orifices of the great veins, and extend for some 

 little distance along the vessels, particularly on the pulmonary 

 veins, which have thick circular muscular coats after they leave 

 the lungs. 



The fibres of the auricles are not directly continuous with those 

 of the ventricles, the auricular and ventricular fibres being only 

 related to each other by their points of origin, viz., the auriculo- 

 ventricular fibrous zones. 



MINUTE STRUCTURE. 



The muscle tissue of the heart differs both in structure and mode 

 of action from the other forms of contractile tissues of the body. 

 The elements are firmly united with one another to form an irreg- 

 ular close network, which, however, can be broken up into masses 

 easily recognizable as peculiar cells. These cells are irregular 

 prismoidal blocks with blunt ends, often split into two to allow of 

 connection with the two contiguous cells. They contain a distinct 

 nucleus, situated in the central axis of the cell. The cells are 

 not surrounded by a distinct sheath of sarcolemma. 



Though striated, like the skeletal muscles, the action of the 

 heart muscle is peculiarly independent of the great nervous centres, 

 being quite involuntary, and characterized by a definite perio- 

 dicity. Its contraction is very slow when compared with the 

 skeletal muscles, but it is much more rapid than that of the con- 

 tracting tissues of most of the hollow viscera. 



VALVES. 



The orifices which lead into and out of the ventricles have 

 peculiar arrangements of their lining texture, forming valves 

 which allow the blood to pass in a certain direction only. These 

 valves, which form a most interesting and important part of the 

 economy of the heart, are of two kinds, differing completely in 

 their mode of action. One kind directs the passage of the blood 

 from the auricles to the ventricles, the other guards the openings 

 into the great arteries. The auriculo-ventricular openings aie 



