264 MANUAL OF PHYSIOLOGY. 



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 irregular 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 sar- 

 colemma. 



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

 heart muscle is peculiarly independent of the great nervous cen- 

 tres, being quite involuntary, and characterized by a definite 

 periodicity. 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 food 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 are 

 protected by valves with a sail-like action. These are made up 

 of delicate curtains formed of thin sheets of connective tissue 

 arising from the margins of the auriculo-ventricular openings, 



