8 4 



HISTOLOGY. 



i 



Capil- 

 laries. 



connected with ground membranes. It may be said that here and there 

 within the cardiac muscle two successive ground membranes are closer 

 together than usual and the fibrils in crossing such an interval become 

 expanded and more stainable, thus making an intercalated disc. The 

 discs have been variously interpreted, for example, as locally contracted 

 segments; as lines where the fibrils are inserted and upon which they may 

 pull in contracting; or as places where the fibrils may grow to form new 

 segments, being comparable with the unbanded embryonic fibrils. The 

 older idea that they are cell boundaries, either cement lines, or protoplasmic 



bridges, is supported by 

 the tendency of heart 

 muscle to rupture along 

 their course. They mark 

 off irregular spaces, how- 

 ever, some containing 

 more than one nucleus, 

 and others non-nucle- 

 ated. Intercalated discs 

 should be distinguished 

 from the cut edges of 

 fiber, made where a 

 branch of the syncytium 

 extending toward the ob- 

 server, passed out of the 

 plane of section. 



ite*- x^xr v The nuclei of cardiac 



muscle are round or oval 

 and are found near the 

 central axes of the fibers. 

 As the fibrils spread out 

 to pass around them, 

 often a considerable quantity of granular protoplasm may be seen, con- 

 taining fat droplets and pigment granules which increase with age. A 

 delicate membrane (sarcolemma) has been described as surrounding the 

 cardiac fibers, and in it the ground and median membranes are said to 

 terminate. Some of the clefts in cardiac muscle are protoplasmic (sarco- 

 plasmic) intervals between bundles of fibrils. Others, bounded by the 

 sarcolemma, are spaces which contain capillary vessels closely applied to 

 the muscle. Probably always a little connective tissue intervenes between 

 the vessel and sarcolemma. The connective tissue, which is more abundant 

 toward the surfaces of the heart, contains tissue spaces and the nerves 



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Endo- Elastic Nucleus Cross Nucleus Nuclei of connec- 

 thelium. fibers. of a sections of a tive tissue cells, 

 connective of muscle 

 tissue muscle fiber, 

 cell. fibers. 



FIG. 100. FROM A CROSS SECTION OF A POPILLARY MUSCLE OP 

 THE HUMAN HEART. X 360. 



