764 HANDBOOK OF PHYSIOLOGY -^ CIRCULATION II 



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 i-alvulac bit u*pidalis 



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fig. 4. Base of the human ventricles seen from their cranial aspect after the atria have been re- 

 moved. The shape of the ostial orifices in the state of contraction differs significantly from the shape 

 in the state of relaxation, as indicated by the dashed lines. [From Spalteholz (148).] 



by elastic rebound at the beginning of muscular 

 relaxation. 



Many strands of myocardial fibers end with 

 tendinous tissues. Yet one cannot compare them with 

 skeletal muscle, since there is no bone to provide a 

 fixed attachment. In reality, all myocardial fibers end 

 on other myocardial fibers either directly or by- 

 insertion of connective tissue. For instance the myo- 

 cardial fibers of the papillary muscles continue as 

 chordae tendineae, which in turn lead via the bicuspid 

 and tricuspid valve leaflets and the fibrous tissue 

 of the heart skeleton to other myocardial fibers. This 

 arrangement forms a circle of myocardial tissue, 

 although with inclusion of a tendinous segment. 

 Other myocardial fibers, such as many strands in the 

 left ventricular deep bulbospiral bundles, simply form 

 a circle. Since, in the final analysis, all myocardial 

 fibers pull directly or indirectly on other myocardial 

 fibers, the concerted effect of their contraction 

 diminishes each heart cavity more or less concentri- 

 cally [see also Hawthorne (67)]. It also stands to 



reason that all muscle "■fiber-rings" which include a 

 noncontractile segment exert during their contraction 

 a pull on the noncontractile segment, storing in it 

 potential energy for release during myocardial 

 relaxation. 



The cells of the conduction system have a special 

 position as far as their participation in the contractile 

 process is concerned. They are derived from muscle 

 cells, but their primary function is the fast conduction 

 of excitation. Yet they do contain a small number of 

 myofibrils and therefore must be expected to partici- 

 pate in the over-all myocardial activity. Since nobody- 

 has measured their contractile force, it remains a 

 matter of conjecture whether Purkinje cells contribute 

 to any significant extent to the force of ventricular 

 contraction. It may be that their contraction serves 

 only the purpose of diminishing the shear forces which 

 would develop between myocardial and Purkinje 

 cells if the latter remain purely passive. In the 

 longitudinal direction the Purkinje cells are joined to 

 intermediate cells which connect them to myocardial 



